{
  "index": "1963-A-6",
  "type": "GEO",
  "tag": [
    "GEO",
    "ANA",
    "ALG"
  ],
  "difficulty": "",
  "question": "6. Let \\( U \\) and \\( V \\) be any two distinct points on an ellipse, let \\( M \\) be the midpoint of the chord \\( U V \\), and let \\( A B \\) and \\( C D \\) be any two other chords through \\( M \\). If the line \\( U V \\) meets the line \\( A C \\) in the point \\( P \\) and the line \\( B D \\) in the point \\( Q \\), prove that \\( M \\) is the midpoint of the segment \\( P Q \\).",
  "solution": "First Solution. Let \\( U V \\) be the \\( x \\)-axis of an oblique coordinate system with \\( M \\) the origin and the \\( y \\) axis distinct from lines \\( A B \\) and \\( C D \\). Suppose \\( y=m x \\) and \\( y=n x \\) are the equations of lines \\( A B \\) and \\( C D \\) respectively. Let \\( a x^{2}+b y^{2}+c x y+d x+e y+f=0 \\) be the equation of the ellipse. Since the ellipse passes through \\( V(t, 0) \\) and \\( U(-t, 0) \\), it follows that \\( d=0 \\).\n\nNow any conic through the four points \\( A, B, C, D \\) can be represented by\n\\[\nk_{1}\\left(a x^{2}+b y^{2}+c x y+e y+f\\right)+k_{2}(y-m x)(y-n x)=0\n\\]\nfor suitable choice of \\( k_{1}, k_{2} \\). Such a conic intersects the \\( x \\)-axis in two points whose \\( x \\) coordinates satisfy \\( k_{1}\\left(\\ldots x^{2}+f\\right)+k_{2} m n x^{2}=0 \\), i.e., in two points symmetric in \\( M \\). In particular the degenerate conic consisting of the lines \\( C A \\) and \\( D B \\) intersects the \\( x \\)-axis in points \\( P \\) and \\( Q \\) which are symmetric relative to \\( M \\) : i.e.. \\( M \\) is the midpoint of \\( P Q \\).\n\nSecond Solution. We treat the problem projectively. We consider all conics through the four points \\( A . B . C \\). \\( D \\). Let \\( R=A C \\cap B D \\) and \\( S= \\) \\( A D \\cap B C \\). Then the polar of the point \\( M \\) with respect to any of the conics is the line \\( R S \\).\n\nLet \\( T=R S \\cap U V \\). Considering the original ellipse we see that \\( U, V \\) are divided harmonically by \\( M \\) and \\( T \\). (Since any chord of a conic is divided harmonically by any point on it and the polar of that point.) Since \\( M \\) is the midpoint of \\( U V \\). it follows that \\( T \\) is on the line at infinity.\n\nLet \\( P=A C \\cap U V=A C \\cap M T \\) and \\( Q=B D \\cap U V=B D \\cap M T \\). Then \\( P Q \\) is a chord of the degenerate conic \\( A C \\cup B D \\) so it is divided harmonically by \\( M \\) and \\( T \\). Since \\( T \\) is at infinity, \\( M \\) is the midpoint of \\( P Q \\).\n\nThird Solution. The following argument avoids explicit consideration of degenerate conics. Let \\( X \\mapsto X^{\\prime} \\) be a projective transformation that carries the given ellipse \\( E \\) into a circle with center \\( M^{\\prime} \\). This is possible since we can first arrange that the polar \\( m \\) of \\( M \\) is transformed into the line at infinity. Then since the line \\( m \\) does not intersect \\( E(M \\) is inside of \\( E), E^{\\prime} \\) is a bounded central conic (i.e., an ellipse) with center \\( M^{\\prime} \\).\nA further transformation (stretching in one direction) will take \\( E^{\\prime} \\) into a circle.\n\nSuppose \\( T \\) is the point at infinity on \\( U V \\). Then \\( U, V ; M, T \\) is a harmonic quadruple. Hence \\( U^{\\prime}, V^{\\prime} ; M^{\\prime}, T^{\\prime} \\) is also a harmonic quadruple. Since \\( M^{\\prime} \\) bisects \\( U^{\\prime} V^{\\prime} \\), it follows that \\( T^{\\prime} \\) is at infinity.\n\nNow \\( A^{\\prime} C^{\\prime} B^{\\prime} D^{\\prime} \\) is a rectangle, since \\( A^{\\prime} B^{\\prime} \\) and \\( C^{\\prime} D^{\\prime} \\) are diameters of the circle \\( E^{\\prime} \\), so it is immediate that \\( M^{\\prime} \\) bisects \\( P^{\\prime} Q^{\\prime} \\). Hence \\( P^{\\prime}, Q^{\\prime} \\); \\( M^{\\prime}, T^{\\prime} \\) is harmonic, so \\( P, Q ; M, T \\) is harmonic. Since \\( T \\) is at infinity, \\( M \\) bisects \\( P Q \\).\n\nRemarks. These proofs show, of course, much more than was asked for. The ellipse can be any conic and the two lines \\( A D \\) and \\( C B \\) can be replaced by any conic through \\( A, B, C, D \\).\n\nThis theorem is well known in the literature of conic sections as the \"butterfly theorem.\" See, for example, A Survey of Geometry, H. Eves, Boston, 1972; also M. S. Klamkin, \"An Extension of the Butterfly Problem,\" Mathematics Magazine, vol. 38 (1965), pages 206-208, and C. D. Chakerian, G. T. Sallee, and M. S. Klamkin, \"The Butterfly Property,\" Mathematics Magazine, 42 (1969).",
  "vars": [
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  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "U": "pointu",
        "V": "pointv",
        "M": "midpoint",
        "A": "pointa",
        "B": "pointb",
        "C": "pointc",
        "D": "pointd",
        "P": "pointp",
        "Q": "pointq",
        "R": "pointr",
        "S": "points",
        "T": "pointt",
        "E": "ellipse",
        "x": "coordx",
        "y": "coordy",
        "m": "slopeab",
        "n": "slopecd",
        "t": "halfdist",
        "a": "coeffaa",
        "b": "coeffbb",
        "c": "coeffcc",
        "d": "coeffdd",
        "e": "coeffee",
        "f": "coeffff",
        "k_1": "paramone",
        "k_2": "paramtwo"
      },
      "question": "6. Let \\( pointu \\) and \\( pointv \\) be any two distinct points on an ellipse, let \\( midpoint \\) be the midpoint of the chord \\( pointu pointv \\), and let \\( pointa pointb \\) and \\( pointc pointd \\) be any two other chords through \\( midpoint \\). If the line \\( pointu pointv \\) meets the line \\( pointa pointc \\) in the point \\( pointp \\) and the line \\( pointb pointd \\) in the point \\( pointq \\), prove that \\( midpoint \\) is the midpoint of the segment \\( pointp pointq \\).",
      "solution": "First Solution. Let \\( pointu pointv \\) be the \\( coordx \\)-axis of an oblique coordinate system with \\( midpoint \\) the origin and the \\( coordy \\) axis distinct from lines \\( pointa pointb \\) and \\( pointc pointd \\). Suppose \\( coordy=slopeab coordx \\) and \\( coordy=slopecd coordx \\) are the equations of lines \\( pointa pointb \\) and \\( pointc pointd \\) respectively. Let \\( coeffaa coordx^{2}+coeffbb coordy^{2}+coeffcc coordx coordy+coeffdd coordx+coeffee coordy+coeffff=0 \\) be the equation of the ellipse. Since the ellipse passes through \\( pointv(halfdist, 0) \\) and \\( pointu(-halfdist, 0) \\), it follows that \\( coeffdd=0 \\).\n\nNow any conic through the four points \\( pointa, pointb, pointc, pointd \\) can be represented by\n\\[\nparamone\\left(coeffaa coordx^{2}+coeffbb coordy^{2}+coeffcc coordx coordy+coeffee coordy+coeffff\\right)+paramtwo(coordy-slopeab coordx)(coordy-slopecd coordx)=0\n\\]\nfor suitable choice of \\( paramone, paramtwo \\). Such a conic intersects the \\( coordx \\)-axis in two points whose \\( coordx \\) coordinates satisfy \\( paramone\\left(\\ldots coordx^{2}+coeffff\\right)+paramtwo slopeab slopecd coordx^{2}=0 \\), i.e., in two points symmetric in \\( midpoint \\). In particular the degenerate conic consisting of the lines \\( pointc pointa \\) and \\( pointd pointb \\) intersects the \\( coordx \\)-axis in points \\( pointp \\) and \\( pointq \\) which are symmetric relative to \\( midpoint \\) : i.e.. \\( midpoint \\) is the midpoint of \\( pointp pointq \\).\n\nSecond Solution. We treat the problem projectively. We consider all conics through the four points \\( pointa . pointb . pointc \\). \\( pointd \\). Let \\( pointr=pointa pointc \\cap pointb pointd \\) and \\( points= \\) \\( pointa pointd \\cap pointb pointc \\). Then the polar of the point \\( midpoint \\) with respect to any of the conics is the line \\( pointr points \\).\n\nLet \\( pointt=pointr points \\cap pointu pointv \\). Considering the original ellipse we see that \\( pointu, pointv \\) are divided harmonically by \\( midpoint \\) and \\( pointt \\). (Since any chord of a conic is divided harmonically by any point on it and the polar of that point.) Since \\( midpoint \\) is the midpoint of \\( pointu pointv \\). it follows that \\( pointt \\) is on the line at infinity.\n\nLet \\( pointp=pointa pointc \\cap pointu pointv=pointa pointc \\cap midpoint pointt \\) and \\( pointq=pointb pointd \\cap pointu pointv=pointb pointd \\cap midpoint pointt \\). Then \\( pointp pointq \\) is a chord of the degenerate conic \\( pointa pointc \\cup pointb pointd \\) so it is divided harmonically by \\( midpoint \\) and \\( pointt \\). Since \\( pointt \\) is at infinity, \\( midpoint \\) is the midpoint of \\( pointp pointq \\).\n\nThird Solution. The following argument avoids explicit consideration of degenerate conics. Let \\( X \\mapsto X^{\\prime} \\) be a projective transformation that carries the given ellipse \\( ellipse \\) into a circle with center \\( midpoint^{\\prime} \\). This is possible since we can first arrange that the polar \\( slopeab \\) of \\( midpoint \\) is transformed into the line at infinity. Then since the line \\( slopeab \\) does not intersect \\( ellipse(midpoint \\) is inside of \\( ellipse), ellipse^{\\prime} \\) is a bounded central conic (i.e., an ellipse) with center \\( midpoint^{\\prime} \\).\nA further transformation (stretching in one direction) will take \\( ellipse^{\\prime} \\) into a circle.\n\nSuppose \\( pointt \\) is the point at infinity on \\( pointu pointv \\). Then \\( pointu, pointv ; midpoint, pointt \\) is a harmonic quadruple. Hence \\( pointu^{\\prime}, pointv^{\\prime} ; midpoint^{\\prime}, pointt^{\\prime} \\) is also a harmonic quadruple. Since \\( midpoint^{\\prime} \\) bisects \\( pointu^{\\prime} pointv^{\\prime} \\), it follows that \\( pointt^{\\prime} \\) is at infinity.\n\nNow \\( pointa^{\\prime} pointc^{\\prime} pointb^{\\prime} pointd^{\\prime} \\) is a rectangle, since \\( pointa^{\\prime} pointb^{\\prime} \\) and \\( pointc^{\\prime} pointd^{\\prime} \\) are diameters of the circle \\( ellipse^{\\prime} \\), so it is immediate that \\( midpoint^{\\prime} \\) bisects \\( pointp^{\\prime} pointq^{\\prime} \\). Hence \\( pointp^{\\prime}, pointq^{\\prime} \\); \\( midpoint^{\\prime}, pointt^{\\prime} \\) is harmonic, so \\( pointp, pointq ; midpoint, pointt \\) is harmonic. Since \\( pointt \\) is at infinity, \\( midpoint \\) bisects \\( pointp pointq \\).\n\nRemarks. These proofs show, of course, much more than was asked for. The ellipse can be any conic and the two lines \\( pointa pointd \\) and \\( pointc pointb \\) can be replaced by any conic through \\( pointa, pointb, pointc, pointd \\).\n\nThis theorem is well known in the literature of conic sections as the \"butterfly theorem.\" See, for example, A Survey of Geometry, H. Eves, Boston, 1972; also M. S. Klamkin, \"An Extension of the Butterfly Problem,\" Mathematics Magazine, vol. 38 (1965), pages 206-208, and C. D. Chakerian, G. T. Sallee, and M. S. Klamkin, \"The Butterfly Property,\" Mathematics Magazine, 42 (1969)."
    },
    "descriptive_long_confusing": {
      "map": {
        "U": "moonstone",
        "V": "riverbank",
        "M": "copperleaf",
        "A": "starlight",
        "B": "shadowmere",
        "C": "pinegrove",
        "D": "silverwing",
        "P": "glasswork",
        "Q": "wildfire",
        "R": "dawnchaser",
        "S": "nightshade",
        "T": "frostglade",
        "E": "marigold",
        "x": "blossomfox",
        "y": "embertrail",
        "m": "lanternfly",
        "n": "willowmist",
        "t": "raincaller",
        "a": "driftcloud",
        "b": "briarwood",
        "c": "goldfinch",
        "d": "thundersky",
        "e": "oceansilk",
        "f": "starflower",
        "k_1": "dreamweave",
        "k_2": "ironquartz"
      },
      "question": "6. Let \\( moonstone \\) and \\( riverbank \\) be any two distinct points on an ellipse, let \\( copperleaf \\) be the midpoint of the chord \\( moonstone riverbank \\), and let \\( starlight shadowmere \\) and \\( pinegrove silverwing \\) be any two other chords through \\( copperleaf \\). If the line \\( moonstone riverbank \\) meets the line \\( starlight pinegrove \\) in the point \\( glasswork \\) and the line \\( shadowmere silverwing \\) in the point \\( wildfire \\), prove that \\( copperleaf \\) is the midpoint of the segment \\( glasswork wildfire \\).",
      "solution": "First Solution. Let \\( moonstone riverbank \\) be the \\( blossomfox \\)-axis of an oblique coordinate system with \\( copperleaf \\) the origin and the \\( embertrail \\) axis distinct from lines \\( starlight shadowmere \\) and \\( pinegrove silverwing \\). Suppose \\( embertrail = lanternfly\\, blossomfox \\) and \\( embertrail = willowmist\\, blossomfox \\) are the equations of lines \\( starlight shadowmere \\) and \\( pinegrove silverwing \\) respectively. Let \\( driftcloud\\, blossomfox^{2}+ briarwood\\, embertrail^{2}+ goldfinch\\, blossomfox\\, embertrail + thundersky\\, blossomfox + oceansilk\\, embertrail + starflower = 0 \\) be the equation of the ellipse. Since the ellipse passes through \\( riverbank(raincaller, 0) \\) and \\( moonstone(-raincaller, 0) \\), it follows that \\( thundersky = 0 \\).\n\nNow any conic through the four points \\( starlight, shadowmere, pinegrove, silverwing \\) can be represented by\n\\[\ndreamweave\\left(driftcloud\\, blossomfox^{2}+ briarwood\\, embertrail^{2}+ goldfinch\\, blossomfox\\, embertrail + oceansilk\\, embertrail + starflower\\right)+ ironquartz (embertrail - lanternfly\\, blossomfox)(embertrail - willowmist\\, blossomfox)=0\n\\]\nfor suitable choice of \\( dreamweave, ironquartz \\). Such a conic intersects the \\( blossomfox \\)-axis in two points whose \\( blossomfox \\) coordinates satisfy \\( dreamweave\\left(\\ldots blossomfox^{2}+ starflower\\right)+ ironquartz\\, lanternfly\\, willowmist\\, blossomfox^{2}=0 \\), i.e., in two points symmetric in \\( copperleaf \\). In particular the degenerate conic consisting of the lines \\( pinegrove starlight \\) and \\( silverwing shadowmere \\) intersects the \\( blossomfox \\)-axis in points \\( glasswork \\) and \\( wildfire \\) which are symmetric relative to \\( copperleaf \\): i.e., \\( copperleaf \\) is the midpoint of \\( glasswork wildfire \\).\n\nSecond Solution. We treat the problem projectively. We consider all conics through the four points \\( starlight . shadowmere . pinegrove . silverwing \\). Let \\( dawnchaser = starlight pinegrove \\cap shadowmere silverwing \\) and \\( nightshade = starlight silverwing \\cap shadowmere pinegrove \\). Then the polar of the point \\( copperleaf \\) with respect to any of the conics is the line \\( dawnchaser nightshade \\).\n\nLet \\( frostglade = dawnchaser nightshade \\cap moonstone riverbank \\). Considering the original ellipse we see that \\( moonstone, riverbank \\) are divided harmonically by \\( copperleaf \\) and \\( frostglade \\). (Since any chord of a conic is divided harmonically by any point on it and the polar of that point.) Since \\( copperleaf \\) is the midpoint of \\( moonstone riverbank \\), it follows that \\( frostglade \\) is on the line at infinity.\n\nLet \\( glasswork = starlight pinegrove \\cap moonstone riverbank = starlight pinegrove \\cap copperleaf frostglade \\) and \\( wildfire = shadowmere silverwing \\cap moonstone riverbank = shadowmere silverwing \\cap copperleaf frostglade \\). Then \\( glasswork wildfire \\) is a chord of the degenerate conic \\( starlight pinegrove \\cup shadowmere silverwing \\) so it is divided harmonically by \\( copperleaf \\) and \\( frostglade \\). Since \\( frostglade \\) is at infinity, \\( copperleaf \\) is the midpoint of \\( glasswork wildfire \\).\n\nThird Solution. The following argument avoids explicit consideration of degenerate conics. Let \\( X \\mapsto X^{\\prime} \\) be a projective transformation that carries the given ellipse \\( marigold \\) into a circle with center \\( copperleaf^{\\prime} \\). This is possible since we can first arrange that the polar \\( lanternfly \\) of \\( copperleaf \\) is transformed into the line at infinity. Then since the line \\( lanternfly \\) does not intersect \\( marigold(copperleaf \\text{ is inside of } marigold), marigold^{\\prime} \\) is a bounded central conic (i.e., an ellipse) with center \\( copperleaf^{\\prime} \\).\nA further transformation (stretching in one direction) will take \\( marigold^{\\prime} \\) into a circle.\n\nSuppose \\( frostglade \\) is the point at infinity on \\( moonstone riverbank \\). Then \\( moonstone, riverbank ; copperleaf, frostglade \\) is a harmonic quadruple. Hence \\( moonstone^{\\prime}, riverbank^{\\prime} ; copperleaf^{\\prime}, frostglade^{\\prime} \\) is also a harmonic quadruple. Since \\( copperleaf^{\\prime} \\) bisects \\( moonstone^{\\prime} riverbank^{\\prime} \\), it follows that \\( frostglade^{\\prime} \\) is at infinity.\n\nNow \\( starlight^{\\prime} pinegrove^{\\prime} shadowmere^{\\prime} silverwing^{\\prime} \\) is a rectangle, since \\( starlight^{\\prime} shadowmere^{\\prime} \\) and \\( pinegrove^{\\prime} silverwing^{\\prime} \\) are diameters of the circle \\( marigold^{\\prime} \\), so it is immediate that \\( copperleaf^{\\prime} \\) bisects \\( glasswork^{\\prime} wildfire^{\\prime} \\). Hence \\( glasswork^{\\prime}, wildfire^{\\prime} ; copperleaf^{\\prime}, frostglade^{\\prime} \\) is harmonic, so \\( glasswork, wildfire ; copperleaf, frostglade \\) is harmonic. Since \\( frostglade \\) is at infinity, \\( copperleaf \\) bisects \\( glasswork wildfire \\).\n\nRemarks. These proofs show, of course, much more than was asked for. The ellipse can be any conic and the two lines \\( starlight silverwing \\) and \\( pinegrove shadowmere \\) can be replaced by any conic through \\( starlight, shadowmere, pinegrove, silverwing \\).\n\nThis theorem is well known in the literature of conic sections as the \"butterfly theorem.\" See, for example, A Survey of Geometry, H. Eves, Boston, 1972; also M. S. Klamkin, \"An Extension of the Butterfly Problem,\" Mathematics Magazine, vol. 38 (1965), pages 206-208, and C. D. Chakerian, G. T. Sallee, and M. S. Klamkin, \"The Butterfly Property,\" Mathematics Magazine, 42 (1969)."
    },
    "descriptive_long_misleading": {
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        "U": "identicalpoint",
        "V": "coincidentpoint",
        "M": "endpoint",
        "A": "interiorpoint",
        "B": "innerpoint",
        "C": "outerpoint",
        "D": "exteriorpoint",
        "P": "parallelpoint",
        "Q": "nonintersectpoint",
        "R": "disjointpoint",
        "S": "separatedpoint",
        "T": "finitepoint",
        "E": "lineobject",
        "x": "verticalvalue",
        "y": "horizontalvalue",
        "m": "nongradient",
        "n": "flatgradient",
        "t": "impropervalue",
        "a": "nonscalar",
        "b": "nonlinear",
        "c": "uncoupled",
        "d": "nonzeroval",
        "e": "variablecoef",
        "f": "nonmember",
        "k_1": "dependentone",
        "k_2": "dependenttwo"
      },
      "question": "6. Let \\( identicalpoint \\) and \\( coincidentpoint \\) be any two distinct points on an ellipse, let \\( endpoint \\) be the midpoint of the chord \\( identicalpoint coincidentpoint \\), and let \\( interiorpoint innerpoint \\) and \\( outerpoint exteriorpoint \\) be any two other chords through \\( endpoint \\). If the line \\( identicalpoint coincidentpoint \\) meets the line \\( interiorpoint outerpoint \\) in the point \\( parallelpoint \\) and the line \\( innerpoint exteriorpoint \\) in the point \\( nonintersectpoint \\), prove that \\( endpoint \\) is the midpoint of the segment \\( parallelpoint nonintersectpoint \\).",
      "solution": "First Solution. Let \\( identicalpoint coincidentpoint \\) be the \\( verticalvalue \\)-axis of an oblique coordinate system with \\( endpoint \\) the origin and the \\( horizontalvalue \\) axis distinct from lines \\( interiorpoint innerpoint \\) and \\( outerpoint exteriorpoint \\). Suppose \\( horizontalvalue=nongradient\\,verticalvalue \\) and \\( horizontalvalue=flatgradient\\,verticalvalue \\) are the equations of lines \\( interiorpoint innerpoint \\) and \\( outerpoint exteriorpoint \\) respectively. Let \n\\( nonscalar\\,verticalvalue^{2}+nonlinear\\,horizontalvalue^{2}+uncoupled\\,verticalvalue\\,horizontalvalue+nonzeroval\\,verticalvalue+variablecoef\\,horizontalvalue+nonmember=0 \\) be the equation of the ellipse. Since the ellipse passes through \\( coincidentpoint(impropervalue,0) \\) and \\( identicalpoint(-impropervalue,0) \\), it follows that \\( nonzeroval=0 \\).\n\nNow any conic through the four points \\( interiorpoint, innerpoint, outerpoint, exteriorpoint \\) can be represented by\n\\[\ndependentone\\left(nonscalar\\,verticalvalue^{2}+nonlinear\\,horizontalvalue^{2}+uncoupled\\,verticalvalue\\,horizontalvalue+variablecoef\\,horizontalvalue+nonmember\\right)+dependenttwo\\,(horizontalvalue-nongradient\\,verticalvalue)(horizontalvalue-flatgradient\\,verticalvalue)=0\n\\]\nfor suitable choice of \\( dependentone, dependenttwo \\). Such a conic intersects the \\( verticalvalue \\)-axis in two points whose \\( verticalvalue \\) coordinates satisfy \\( dependentone(\\ldots verticalvalue^{2}+nonmember)+dependenttwo\\,nongradient\\,flatgradient\\,verticalvalue^{2}=0 \\), i.e., in two points symmetric in \\( endpoint \\). In particular the degenerate conic consisting of the lines \\( outerpoint interiorpoint \\) and \\( exteriorpoint innerpoint \\) intersects the \\( verticalvalue \\)-axis in points \\( parallelpoint \\) and \\( nonintersectpoint \\) which are symmetric relative to \\( endpoint \\); i.e., \\( endpoint \\) is the midpoint of \\( parallelpoint nonintersectpoint \\).\n\nSecond Solution. We treat the problem projectively. We consider all conics through the four points \\( interiorpoint , innerpoint , outerpoint , exteriorpoint \\). Let \\( disjointpoint = interiorpoint outerpoint \\cap innerpoint exteriorpoint \\) and \\( separatedpoint = interiorpoint exteriorpoint \\cap innerpoint outerpoint \\). Then the polar of the point \\( endpoint \\) with respect to any of the conics is the line \\( disjointpoint separatedpoint \\).\n\nLet \\( finitepoint = disjointpoint separatedpoint \\cap identicalpoint coincidentpoint \\). Considering the original ellipse we see that \\( identicalpoint, coincidentpoint \\) are divided harmonically by \\( endpoint \\) and \\( finitepoint \\). (Since any chord of a conic is divided harmonically by any point on it and the polar of that point.) Since \\( endpoint \\) is the midpoint of \\( identicalpoint coincidentpoint \\), it follows that \\( finitepoint \\) is on the line at infinity.\n\nLet \\( parallelpoint = interiorpoint outerpoint \\cap identicalpoint coincidentpoint = interiorpoint outerpoint \\cap endpoint finitepoint \\) and \\( nonintersectpoint = innerpoint exteriorpoint \\cap identicalpoint coincidentpoint = innerpoint exteriorpoint \\cap endpoint finitepoint \\). Then \\( parallelpoint nonintersectpoint \\) is a chord of the degenerate conic \\( interiorpoint outerpoint \\cup innerpoint exteriorpoint \\) so it is divided harmonically by \\( endpoint \\) and \\( finitepoint \\). Since \\( finitepoint \\) is at infinity, \\( endpoint \\) is the midpoint of \\( parallelpoint nonintersectpoint \\).\n\nThird Solution. The following argument avoids explicit consideration of degenerate conics. Let \\( X \\mapsto X^{\\prime} \\) be a projective transformation that carries the given ellipse \\( lineobject \\) into a circle with center \\( endpoint^{\\prime} \\). This is possible since we can first arrange that the polar \\( nongradient \\) of \\( endpoint \\) is transformed into the line at infinity. Then since the line \\( nongradient \\) does not intersect \\( lineobject(endpoint \\) is inside of \\( lineobject), lineobject^{\\prime} \\) is a bounded central conic (i.e., an ellipse) with center \\( endpoint^{\\prime} \\).\nA further transformation (stretching in one direction) will take \\( lineobject^{\\prime} \\) into a circle.\n\nSuppose \\( finitepoint \\) is the point at infinity on \\( identicalpoint coincidentpoint \\). Then \\( identicalpoint, coincidentpoint ; endpoint, finitepoint \\) is a harmonic quadruple. Hence \\( identicalpoint^{\\prime}, coincidentpoint^{\\prime} ; endpoint^{\\prime}, finitepoint^{\\prime} \\) is also a harmonic quadruple. Since \\( endpoint^{\\prime} \\) bisects \\( identicalpoint^{\\prime} coincidentpoint^{\\prime} \\), it follows that \\( finitepoint^{\\prime} \\) is at infinity.\n\nNow \\( interiorpoint^{\\prime} outerpoint^{\\prime} innerpoint^{\\prime} exteriorpoint^{\\prime} \\) is a rectangle, since \\( interiorpoint^{\\prime} innerpoint^{\\prime} \\) and \\( outerpoint^{\\prime} exteriorpoint^{\\prime} \\) are diameters of the circle \\( lineobject^{\\prime} \\), so it is immediate that \\( endpoint^{\\prime} \\) bisects \\( parallelpoint^{\\prime} nonintersectpoint^{\\prime} \\). Hence \\( parallelpoint^{\\prime}, nonintersectpoint^{\\prime} ; endpoint^{\\prime}, finitepoint^{\\prime} \\) is harmonic, so \\( parallelpoint, nonintersectpoint ; endpoint, finitepoint \\) is harmonic. Since \\( finitepoint \\) is at infinity, \\( endpoint \\) bisects \\( parallelpoint nonintersectpoint \\).\n\nRemarks. These proofs show, of course, much more than was asked for. The ellipse can be any conic and the two lines \\( interiorpoint exteriorpoint \\) and \\( outerpoint innerpoint \\) can be replaced by any conic through \\( interiorpoint, innerpoint, outerpoint, exteriorpoint \\).\n\nThis theorem is well known in the literature of conic sections as the \"butterfly theorem.\" See, for example, A Survey of Geometry, H. Eves, Boston, 1972; also M. S. Klamkin, \"An Extension of the Butterfly Problem,\" Mathematics Magazine, vol. 38 (1965), pages 206-208, and C. D. Chakerian, G. T. Sallee, and M. S. Klamkin, \"The Butterfly Property,\" Mathematics Magazine, 42 (1969)."
    },
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        "U": "slkdhfma",
        "V": "gjpoynrb",
        "M": "zcqtrvae",
        "A": "vypqklsd",
        "B": "wsnrehfj",
        "C": "hdgkimzb",
        "D": "rnbvdxye",
        "P": "qrstumle",
        "Q": "kvpaocht",
        "R": "jzwekdhs",
        "S": "hplcmdru",
        "T": "yvgtsfqc",
        "E": "xcnvbrla",
        "x": "onpqjdrm",
        "y": "lekvozsa",
        "m": "jaezywqt",
        "n": "fvlixsop",
        "t": "kgqhrdub",
        "a": "sbgilewp",
        "b": "rsejkhdf",
        "c": "fjznouya",
        "d": "tymklhvg",
        "f": "vwyabcnq",
        "k_1": "oxszpgda",
        "k_2": "qdhpmlco"
      },
      "question": "6. Let \\( slkdhfma \\) and \\( gjpoynrb \\) be any two distinct points on an ellipse, let \\( zcqtrvae \\) be the midpoint of the chord \\( slkdhfma gjpoynrb \\), and let \\( vypqklsd wsnrehfj \\) and \\( hdgkimzb rnbvdxye \\) be any two other chords through \\( zcqtrvae \\). If the line \\( slkdhfma gjpoynrb \\) meets the line \\( vypqklsd hdgkimzb \\) in the point \\( qrstumle \\) and the line \\( wsnrehfj rnbvdxye \\) in the point \\( kvpaocht \\), prove that \\( zcqtrvae \\) is the midpoint of the segment \\( qrstumle kvpaocht \\).",
      "solution": "First Solution. Let \\( slkdhfma gjpoynrb \\) be the \\( onpqjdrm \\)-axis of an oblique coordinate system with \\( zcqtrvae \\) the origin and the \\( lekvozsa \\) axis distinct from lines \\( vypqklsd wsnrehfj \\) and \\( hdgkimzb rnbvdxye \\). Suppose \\( lekvozsa=jaezywqt\\,onpqjdrm \\) and \\( lekvozsa=fvlixsop\\,onpqjdrm \\) are the equations of lines \\( vypqklsd wsnrehfj \\) and \\( hdgkimzb rnbvdxye \\) respectively. Let \\( sbgilewp\\,onpqjdrm^{2}+rsejkhdf\\,lekvozsa^{2}+fjznouya\\,onpqjdrm\\,lekvozsa+tymklhvg\\,onpqjdrm+e\\,lekvozsa+vwyabcnq=0 \\) be the equation of the ellipse. Since the ellipse passes through \\( gjpoynrb(kgqhrdub,0) \\) and \\( slkdhfma(-kgqhrdub,0) \\), it follows that \\( tymklhvg=0 \\).\n\nNow any conic through the four points \\( vypqklsd, wsnrehfj, hdgkimzb, rnbvdxye \\) can be represented by\n\\[\noxszpgda\\left(sbgilewp\\,onpqjdrm^{2}+rsejkhdf\\,lekvozsa^{2}+fjznouya\\,onpqjdrm\\,lekvozsa+e\\,lekvozsa+vwyabcnq\\right)+qdhpmlco(lekvozsa-jaezywqt\\,onpqjdrm)(lekvozsa-fvlixsop\\,onpqjdrm)=0\n\\]\nfor suitable choice of \\( oxszpgda, qdhpmlco \\). Such a conic intersects the \\( onpqjdrm \\)-axis in two points whose \\( onpqjdrm \\) coordinates satisfy \\( oxszpgda(\\ldots onpqjdrm^{2}+vwyabcnq)+qdhpmlco\\,jaezywqt\\,fvlixsop\\,onpqjdrm^{2}=0 \\), i.e., in two points symmetric in \\( zcqtrvae \\). In particular the degenerate conic consisting of the lines \\( hdgkimzb vypqklsd \\) and \\( rnbvdxye wsnrehfj \\) intersects the \\( onpqjdrm \\)-axis in points \\( qrstumle \\) and \\( kvpaocht \\) which are symmetric relative to \\( zcqtrvae \\); i.e., \\( zcqtrvae \\) is the midpoint of \\( qrstumle kvpaocht \\).\n\nSecond Solution. We treat the problem projectively. We consider all conics through the four points \\( vypqklsd, wsnrehfj, hdgkimzb, rnbvdxye \\). Let \\( jzwekdhs=vypqklsd hdgkimzb \\cap wsnrehfj rnbvdxye \\) and \\( hplcmdru=vypqklsd rnbvdxye \\cap wsnrehfj hdgkimzb \\). Then the polar of the point \\( zcqtrvae \\) with respect to any of the conics is the line \\( jzwekdhs hplcmdru \\).\n\nLet \\( yvgtsfqc=jzwekdhs hplcmdru \\cap slkdhfma gjpoynrb \\). Considering the original ellipse we see that \\( slkdhfma, gjpoynrb \\) are divided harmonically by \\( zcqtrvae \\) and \\( yvgtsfqc \\). (Since any chord of a conic is divided harmonically by any point on it and the polar of that point.) Since \\( zcqtrvae \\) is the midpoint of \\( slkdhfma gjpoynrb \\), it follows that \\( yvgtsfqc \\) is on the line at infinity.\n\nLet \\( qrstumle=vypqklsd hdgkimzb \\cap slkdhfma gjpoynrb = vypqklsd hdgkimzb \\cap zcqtrvae yvgtsfqc \\) and \\( kvpaocht=wsnrehfj rnbvdxye \\cap slkdhfma gjpoynrb = wsnrehfj rnbvdxye \\cap zcqtrvae yvgtsfqc \\). Then \\( qrstumle kvpaocht \\) is a chord of the degenerate conic \\( vypqklsd hdgkimzb \\cup wsnrehfj rnbvdxye \\) so it is divided harmonically by \\( zcqtrvae \\) and \\( yvgtsfqc \\). Since \\( yvgtsfqc \\) is at infinity, \\( zcqtrvae \\) is the midpoint of \\( qrstumle kvpaocht \\).\n\nThird Solution. The following argument avoids explicit consideration of degenerate conics. Let \\( X \\mapsto X^{\\prime} \\) be a projective transformation that carries the given ellipse \\( xcnvbrla \\) into a circle with center \\( zcqtrvae^{\\prime} \\). This is possible since we can first arrange that the polar \\( jaezywqt \\) of \\( zcqtrvae \\) is transformed into the line at infinity. Then since the line \\( jaezywqt \\) does not intersect \\( xcnvbrla(zcqtrvae \\) is inside of \\( xcnvbrla), xcnvbrla^{\\prime} \\) is a bounded central conic (i.e., an ellipse) with center \\( zcqtrvae^{\\prime} \\).\nA further transformation (stretching in one direction) will take \\( xcnvbrla^{\\prime} \\) into a circle.\n\nSuppose \\( yvgtsfqc \\) is the point at infinity on \\( slkdhfma gjpoynrb \\). Then \\( slkdhfma, gjpoynrb ; zcqtrvae, yvgtsfqc \\) is a harmonic quadruple. Hence \\( slkdhfma^{\\prime}, gjpoynrb^{\\prime} ; zcqtrvae^{\\prime}, yvgtsfqc^{\\prime} \\) is also a harmonic quadruple. Since \\( zcqtrvae^{\\prime} \\) bisects \\( slkdhfma^{\\prime} gjpoynrb^{\\prime} \\), it follows that \\( yvgtsfqc^{\\prime} \\) is at infinity.\n\nNow \\( vypqklsd^{\\prime} hdgkimzb^{\\prime} wsnrehfj^{\\prime} rnbvdxye^{\\prime} \\) is a rectangle, since \\( vypqklsd^{\\prime} wsnrehfj^{\\prime} \\) and \\( hdgkimzb^{\\prime} rnbvdxye^{\\prime} \\) are diameters of the circle \\( xcnvbrla^{\\prime} \\), so it is immediate that \\( zcqtrvae^{\\prime} \\) bisects \\( qrstumle^{\\prime} kvpaocht^{\\prime} \\). Hence \\( qrstumle^{\\prime}, kvpaocht^{\\prime}; zcqtrvae^{\\prime}, yvgtsfqc^{\\prime} \\) is harmonic, so \\( qrstumle, kvpaocht ; zcqtrvae, yvgtsfqc \\) is harmonic. Since \\( yvgtsfqc \\) is at infinity, \\( zcqtrvae \\) bisects \\( qrstumle kvpaocht \\).\n\nRemarks. These proofs show, of course, much more than was asked for. The ellipse can be any conic and the two lines \\( vypqklsd rnbvdxye \\) and \\( hdgkimzb wsnrehfj \\) can be replaced by any conic through \\( vypqklsd, wsnrehfj, hdgkimzb, rnbvdxye \\).\n\nThis theorem is well known in the literature of conic sections as the \"butterfly theorem.\" See, for example, A Survey of Geometry, H. Eves, Boston, 1972; also M. S. Klamkin, \"An Extension of the Butterfly Problem,\" Mathematics Magazine, vol. 38 (1965), pages 206-208, and C. D. Chakerian, G. T. Sallee, and M. S. Klamkin, \"The Butterfly Property,\" Mathematics Magazine, 42 (1969)."
    },
    "kernel_variant": {
      "question": "Let 𝓗 be a non–degenerate hyperbola and let U , V be two distinct points that lie on the same branch of 𝓗.  Denote by M the (Euclidean) midpoint of the chord UV.  Through M draw two further chords AB and CD, and assume that none of the four lines AB, CD, AC, BD is parallel to UV (so that every forthcoming intersection with UV is a finite point).  Put\n   P = AC ∩ UV,  Q = BD ∩ UV.\nProve that M is the midpoint of the segment PQ.",
      "solution": "Projective–geometric proof (it works for any non-degenerate conic; we merely keep the wording \"hyperbola\" of the problem).\n\n1.  A complete quadrilateral.\n   Since both chords AB and CD pass through M we have M = AB ∩ CD.  Define\n        R = AC ∩ BD,  S = AD ∩ BC.\n   The six lines AB, BC, CD, DA, AC, BD therefore form a complete quadrilateral; the three points M, R, S obtained by intersecting opposite sides are its diagonal points.\n\n2.  The polar of M is fixed inside the pencil of conics through A, B, C, D.\n   Let Γ be the pencil of all conics through the four fixed points A, B, C, D.  By the Quadrilateral–conic theorem (see Coxeter, Projective Geometry, §10.3) the polar of the diagonal point M with respect to every conic 𝒞 ∈ Γ is the same line RS; that is\n        pol𝒞(M) = RS  for every 𝒞 ∈ Γ.\n\n3.  The point T = RS ∩ UV for the given hyperbola 𝓗.\n   Apply Step 2 to the original hyperbola 𝓗.  The line UV passes through M and intersects 𝓗 in the two points U and V, so the chord–polar theorem gives the harmonic relation\n        (U, V; M, T) = –1, where T = RS ∩ UV.\n   Because M is the Euclidean midpoint of U and V, the only way in which UV can be divided harmonically with M in the middle is that T be the point at infinity on the line UV (the other harmonic conjugate of the midpoint).  Consequently\n        T is the point at infinity of the direction UV.\n\n4.  A degenerate member of the pencil.\n   Inside the same pencil Γ choose the degenerate conic Δ := AC ∪ BD.  By Step 2 the polar of M with respect to Δ is again RS, so RS meets UV in the same point T, still the point at infinity.  The line UV meets Δ in the two finite points\n        P = AC ∩ UV,  Q = BD ∩ UV,\n   hence PQ is a chord of the (degenerate) conic Δ.\n\n5.  Harmonic division along UV.\n   For any (possibly degenerate) conic and any point X, the chord–polar theorem states that if a line through X meets the conic at two points P, Q and if Y is where the polar of X meets that line, then (P, Q; X, Y) = –1.  Apply this with\n        X = M,  line = UV,  conic = Δ,  Y = T.\n   The hypotheses are satisfied: UV contains M and meets Δ in P and Q, while the polar of M is RS and meets UV in T.  Therefore\n        (P, Q; M, T) = –1.\n   But T is the point at infinity on UV, so this harmonic condition forces M to be the midpoint of the finite segment PQ.  (Indeed, on an affine line whose point-at-infinity is T, the equality (X, Y; M, T) = –1 translates to M being the Euclidean midpoint of XY.)\n\nHence M bisects PQ, as was to be shown.",
      "_meta": {
        "core_steps": [
          "Consider the pencil of all conics through A,B,C,D (which contains both the given conic and a degenerate member).",
          "For every conic in that pencil, the polar of M is the fixed line RS, where R = AC ∩ BD and S = AD ∩ BC.",
          "Let T = RS ∩ UV; on the original conic U,V,M,T form a harmonic range, and because M is the midpoint of UV this forces T to be the point at infinity on UV.",
          "Take the degenerate conic AC ∪ BD in the pencil; its chord with UV is PQ, where P = AC ∩ UV and Q = BD ∩ UV.",
          "Since M,T still harmonically divide P,Q and T is at infinity, M must be the midpoint of PQ."
        ],
        "mutable_slots": {
          "slot1": {
            "description": "Nature of the given curve: it need only be a non-degenerate conic; ‘ellipse’ is not essential.",
            "original": "ellipse"
          },
          "slot2": {
            "description": "Choice of the particular degenerate conic in the pencil; AC ∪ BD can be replaced by any conic (degenerate or not) through A,B,C,D that meets UV in two points.",
            "original": "degenerate conic AC ∪ BD"
          }
        }
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}