{
  "index": "1970-B-6",
  "type": "GEO",
  "tag": [
    "GEO",
    "ALG"
  ],
  "difficulty": "",
  "question": "B-6. A quadrilateral which can be inscribed in a circle is said to be inscribable or cyclic. A quadrilateral which can be circumscribed to a circle is said to be circumscribable. Show that if a circumscribable quadrilateral of sides \\( a, b, c, d \\) has area \\( A=\\sqrt{a b c d} \\), then it is also inscribable.",
  "solution": "B-6 Since the quadrilateral is circumscribable, \\( a+c=b+d \\). Let \\( k \\) be the length of a diagonal and angles \\( \\alpha \\) and \\( \\beta \\) selected so that \\( k^{2}=a^{2}+b^{2}-2 a b \\cos \\alpha \\) \\( =c^{2}+d^{2}-2 c d \\cos \\beta \\). If we subtract \\( (a-b)^{2}=(c-d)^{2} \\), we obtain\n\\[\n2 a b(1-\\cos \\alpha)=2 c d(1-\\cos \\beta)\n\\]\n\nFrom \\( A=\\frac{1}{2} a b \\sin \\alpha+\\frac{1}{2} c d \\sin \\beta=\\sqrt{a b c d} \\),\n\\[\n4 A^{2}=4 a b c d=a^{2} b^{2}\\left(1-\\cos ^{2} \\alpha\\right)+c^{2} d^{2}\\left(1-\\cos ^{2} \\beta\\right)+2 a b c d \\sin \\alpha \\sin \\beta\n\\]\n\nUsing (1) twice on the right hand side,\n\\[\n\\begin{array}{l}\n4 a b c d=a b(1+\\cos \\alpha) c d(1-\\cos \\beta)+c d(1+\\cos \\beta) a b(1-\\cos \\alpha) \\\\\n+2 a b c d \\sin \\alpha \\sin \\beta .\n\\end{array}\n\\]\n\nOn simplifying, \\( 4=2-2 \\cos (\\alpha+\\beta) \\), which implies that \\( \\alpha+\\beta=\\pi \\) and so the quadrilateral is cyclic.",
  "vars": [
    "k",
    "\\\\alpha",
    "\\\\beta"
  ],
  "params": [
    "a",
    "b",
    "c",
    "d",
    "A"
  ],
  "sci_consts": [],
  "variants": {
    "descriptive_long": {
      "map": {
        "k": "diaglen",
        "\\alpha": "angleone",
        "\\beta": "angletwo",
        "a": "sidefirst",
        "b": "sidesecond",
        "c": "sidethird",
        "d": "sidefourth",
        "A": "areaquad"
      },
      "question": "B-6. A quadrilateral which can be inscribed in a circle is said to be inscribable or cyclic. A quadrilateral which can be circumscribed to a circle is said to be circumscribable. Show that if a circumscribable quadrilateral of sides \\( sidefirst, sidesecond, sidethird, sidefourth \\) has area \\( areaquad=\\sqrt{sidefirst sidesecond sidethird sidefourth} \\), then it is also inscribable.",
      "solution": "B-6 Since the quadrilateral is circumscribable, \\( sidefirst+sidethird=sidesecond+sidefourth \\). Let \\( diaglen \\) be the length of a diagonal and angles \\( angleone \\) and \\( angletwo \\) selected so that \\( diaglen^{2}=sidefirst^{2}+sidesecond^{2}-2 sidefirst sidesecond \\cos angleone \\) \\( =sidethird^{2}+sidefourth^{2}-2 sidethird sidefourth \\cos angletwo \\). If we subtract \\( (sidefirst-sidesecond)^{2}=(sidethird-sidefourth)^{2} \\), we obtain\n\\[\n2 sidefirst sidesecond(1-\\cos angleone)=2 sidethird sidefourth(1-\\cos angletwo)\n\\]\n\nFrom \\( areaquad=\\frac{1}{2} sidefirst sidesecond \\sin angleone+\\frac{1}{2} sidethird sidefourth \\sin angletwo=\\sqrt{sidefirst sidesecond sidethird sidefourth} \\),\n\\[\n4 areaquad^{2}=4 sidefirst sidesecond sidethird sidefourth=sidefirst^{2} sidesecond^{2}\\left(1-\\cos ^{2} angleone\\right)+sidethird^{2} sidefourth^{2}\\left(1-\\cos ^{2} angletwo\\right)+2 sidefirst sidesecond sidethird sidefourth \\sin angleone \\sin angletwo\n\\]\n\nUsing (1) twice on the right hand side,\n\\[\n\\begin{array}{l}\n4 sidefirst sidesecond sidethird sidefourth=sidefirst sidesecond(1+\\cos angleone) sidethird sidefourth(1-\\cos angletwo)+sidethird sidefourth(1+\\cos angletwo) sidefirst sidesecond(1-\\cos angleone) \\\\\n+2 sidefirst sidesecond sidethird sidefourth \\sin angleone \\sin angletwo .\n\\end{array}\n\\]\n\nOn simplifying, \\( 4=2-2 \\cos (angleone+angletwo) \\), which implies that \\( angleone+angletwo=\\pi \\) and so the quadrilateral is cyclic."
    },
    "descriptive_long_confusing": {
      "map": {
        "a": "riverbank",
        "b": "gemstone",
        "c": "sunlight",
        "d": "waterfall",
        "A": "compassrose",
        "k": "breadcrumb",
        "\\alpha": "\\marigold",
        "\\beta": "\\dragonfly"
      },
      "question": "B-6. A quadrilateral which can be inscribed in a circle is said to be inscribable or cyclic. A quadrilateral which can be circumscribed to a circle is said to be circumscribable. Show that if a circumscribable quadrilateral of sides \\( riverbank, gemstone, sunlight, waterfall \\) has area \\( compassrose=\\sqrt{riverbank gemstone sunlight waterfall} \\), then it is also inscribable.",
      "solution": "B-6 Since the quadrilateral is circumscribable, \\( riverbank+sunlight=gemstone+waterfall \\). Let \\( breadcrumb \\) be the length of a diagonal and angles \\( \\marigold \\) and \\( \\dragonfly \\) selected so that \\( breadcrumb^{2}=riverbank^{2}+gemstone^{2}-2 riverbank gemstone \\cos \\marigold \\) \\( =sunlight^{2}+waterfall^{2}-2 sunlight waterfall \\cos \\dragonfly \\). If we subtract \\( (riverbank-gemstone)^{2}=(sunlight-waterfall)^{2} \\), we obtain\n\\[\n2 riverbank gemstone(1-\\cos \\marigold)=2 sunlight waterfall(1-\\cos \\dragonfly)\n\\]\n\nFrom \\( compassrose=\\frac{1}{2} riverbank gemstone \\sin \\marigold+\\frac{1}{2} sunlight waterfall \\sin \\dragonfly=\\sqrt{riverbank gemstone sunlight waterfall} \\),\n\\[\n4 compassrose^{2}=4 riverbank gemstone sunlight waterfall=riverbank^{2} gemstone^{2}\\left(1-\\cos ^{2} \\marigold\\right)+sunlight^{2} waterfall^{2}\\left(1-\\cos ^{2} \\dragonfly\\right)+2 riverbank gemstone sunlight waterfall \\sin \\marigold \\sin \\dragonfly\n\\]\n\nUsing (1) twice on the right hand side,\n\\[\n\\begin{array}{l}\n4 riverbank gemstone sunlight waterfall=riverbank gemstone(1+\\cos \\marigold) sunlight waterfall(1-\\cos \\dragonfly)+sunlight waterfall(1+\\cos \\dragonfly) riverbank gemstone(1-\\cos \\marigold) \\\\\n+2 riverbank gemstone sunlight waterfall \\sin \\marigold \\sin \\dragonfly .\n\\end{array}\n\\]\n\nOn simplifying, \\( 4=2-2 \\cos (\\marigold+\\dragonfly) \\), which implies that \\( \\marigold+\\dragonfly=\\pi \\) and so the quadrilateral is cyclic."
    },
    "descriptive_long_misleading": {
      "map": {
        "a": "centerlength",
        "b": "peripherylen",
        "c": "internalside",
        "d": "externalline",
        "A": "microarea",
        "k": "minordiag",
        "\\alpha": "zeroangle",
        "\\beta": "flatangle"
      },
      "question": "B-6. A quadrilateral which can be inscribed in a circle is said to be inscribable or cyclic. A quadrilateral which can be circumscribed to a circle is said to be circumscribable. Show that if a circumscribable quadrilateral of sides \\( centerlength, peripherylen, internalside, externalline \\) has area \\( microarea=\\sqrt{ centerlength\\, peripherylen\\, internalside\\, externalline } \\), then it is also inscribable.",
      "solution": "B-6 Since the quadrilateral is circumscribable, \\( centerlength+internalside=peripherylen+externalline \\). Let \\( minordiag \\) be the length of a diagonal and angles \\( zeroangle \\) and \\( flatangle \\) selected so that \\( minordiag^{2}=centerlength^{2}+peripherylen^{2}-2\\, centerlength\\, peripherylen \\cos zeroangle = internalside^{2}+externalline^{2}-2\\, internalside\\, externalline \\cos flatangle \\). If we subtract \\( (centerlength-peripherylen)^{2}=(internalside-externalline)^{2} \\), we obtain\n\\[\n2\\, centerlength\\, peripherylen (1-\\cos zeroangle)=2\\, internalside\\, externalline (1-\\cos flatangle)\n\\]\nFrom \\( microarea=\\frac{1}{2}\\, centerlength\\, peripherylen \\sin zeroangle + \\frac{1}{2}\\, internalside\\, externalline \\sin flatangle = \\sqrt{ centerlength\\, peripherylen\\, internalside\\, externalline } \\),\n\\[\n4\\, microarea^{2}=4\\, centerlength\\, peripherylen\\, internalside\\, externalline = centerlength^{2} peripherylen^{2}\\left(1-\\cos ^{2} zeroangle\\right)+internalside^{2} externalline^{2}\\left(1-\\cos ^{2} flatangle\\right)+2\\, centerlength\\, peripherylen\\, internalside\\, externalline \\sin zeroangle \\sin flatangle\n\\]\nUsing (1) twice on the right hand side,\n\\[\n\\begin{array}{l}\n4\\, centerlength\\, peripherylen\\, internalside\\, externalline = centerlength\\, peripherylen (1+\\cos zeroangle)\\; internalside\\, externalline (1-\\cos flatangle) \\\\\n+ internalside\\, externalline (1+\\cos flatangle)\\; centerlength\\, peripherylen (1-\\cos zeroangle) \\\\\n+2\\, centerlength\\, peripherylen\\, internalside\\, externalline \\sin zeroangle \\sin flatangle .\n\\end{array}\n\\]\n\nOn simplifying, \\( 4=2-2 \\cos (zeroangle+flatangle) \\), which implies that \\( zeroangle+flatangle=\\pi \\) and so the quadrilateral is cyclic."
    },
    "garbled_string": {
      "map": {
        "k": "slvjhqne",
        "\\alpha": "gnahxwqe",
        "\\beta": "pzicokld",
        "a": "ryotmsuv",
        "b": "hglafptk",
        "c": "qdevnslu",
        "d": "wmjarbzi",
        "A": "bjivnqeu"
      },
      "question": "B-6. A quadrilateral which can be inscribed in a circle is said to be inscribable or cyclic. A quadrilateral which can be circumscribed to a circle is said to be circumscribable. Show that if a circumscribable quadrilateral of sides \\( ryotmsuv, hglafptk, qdevnslu, wmjarbzi \\) has area \\( bjivnqeu=\\sqrt{ ryotmsuv hglafptk qdevnslu wmjarbzi }\\), then it is also inscribable.",
      "solution": "B-6 Since the quadrilateral is circumscribable, \\( ryotmsuv+qdevnslu=hglafptk+wmjarbzi \\). Let \\( slvjhqne \\) be the length of a diagonal and angles \\( gnahxwqe \\) and \\( pzicokld \\) selected so that \\( slvjhqne^{2}=ryotmsuv^{2}+hglafptk^{2}-2 ryotmsuv hglafptk \\cos gnahxwqe \\) \\( =qdevnslu^{2}+wmjarbzi^{2}-2 qdevnslu wmjarbzi \\cos pzicokld \\). If we subtract \\( (ryotmsuv-hglafptk)^{2}=(qdevnslu-wmjarbzi)^{2} \\), we obtain\n\\[\n2 ryotmsuv hglafptk(1-\\cos gnahxwqe)=2 qdevnslu wmjarbzi(1-\\cos pzicokld)\n\\]\n\nFrom \\( bjivnqeu=\\frac{1}{2} ryotmsuv hglafptk \\sin gnahxwqe+\\frac{1}{2} qdevnslu wmjarbzi \\sin pzicokld=\\sqrt{ ryotmsuv hglafptk qdevnslu wmjarbzi } \\),\n\\[\n4 bjivnqeu^{2}=4 ryotmsuv hglafptk qdevnslu wmjarbzi=ryotmsuv^{2} hglafptk^{2}\\left(1-\\cos ^{2} gnahxwqe\\right)+qdevnslu^{2} wmjarbzi^{2}\\left(1-\\cos ^{2} pzicokld\\right)+2 ryotmsuv hglafptk qdevnslu wmjarbzi \\sin gnahxwqe \\sin pzicokld\n\\]\n\nUsing (1) twice on the right hand side,\n\\[\n\\begin{array}{l}\n4 ryotmsuv hglafptk qdevnslu wmjarbzi=ryotmsuv hglafptk(1+\\cos gnahxwqe) qdevnslu wmjarbzi(1-\\cos pzicokld)+qdevnslu wmjarbzi(1+\\cos pzicokld) ryotmsuv hglafptk(1-\\cos gnahxwqe) \\\\\n+2 ryotmsuv hglafptk qdevnslu wmjarbzi \\sin gnahxwqe \\sin pzicokld .\n\\end{array}\n\\]\n\nOn simplifying, \\( 4=2-2 \\cos (gnahxwqe+pzicokld) \\), which implies that \\( gnahxwqe+pzicokld=\\pi \\) and so the quadrilateral is cyclic."
    },
    "kernel_variant": {
      "question": "Let $ABCD$ be a convex \\emph{tangential} quadrilateral, i.e. $ABCD$ possesses an inscribed circle.  \nDenote its consecutive side–lengths, the two diagonals and its area by  \n\\[\nAB=a,\\; BC=b,\\; CD=c,\\; DA=d,\\qquad AC=m,\\; BD=n,\\qquad \\Delta .\n\\]\n\nAssume that the following two algebraic identities hold  \n\\[\n\\tag{$\\dagger$}\\Delta=\\sqrt{a\\,b\\,c\\,d},\\qquad \n\\tag{$\\ddagger$}(m^{2}+n^{2})^{2}=4\\,(a\\,c+b\\,d)^{2}.\n\\]\n\n(a)  Prove that $ABCD$ is cyclic, i.e. $\\angle A+\\angle C=\\pi$.  \n\n(b)  Prove that the diagonals are equal: $AC=BD$.  \n\n(c)  Conversely, prove that every bicentric quadrilateral (simultaneously tangential \\emph{and} cyclic) whose diagonals are equal fulfils both $(\\dagger)$ and $(\\ddagger)$.",
      "solution": "Throughout we assume $a,b,c,d>0$ and label the vertices in counter–clockwise order.\n\n\\textbf{Step 0.  Preparatory identities for the diagonal $AC=m$.}\n\nBecause $ABCD$ is tangential, Pitot’s theorem gives  \n\\[\n\\tag{0.1} a+c=b+d\n\\quad\\Longrightarrow\\quad\na-b=d-c\n\\quad\\Longrightarrow\\quad\n(a-b)^{2}=(c-d)^{2}.\n\\]\n\nLet  \n\\[\n\\tag{0.2}\\alpha=\\angle ABC,\\qquad \\beta=\\angle CDA ,\n\\]\nso $\\alpha$ and $\\beta$ are opposite angles of $ABCD$ both subtending the diagonal $AC$.\n\nApplying the Law of Cosines in $\\triangle ABC$ and $\\triangle CDA$ we obtain  \n\\[\n\\tag{0.3} m^{2}=a^{2}+b^{2}-2ab\\cos\\alpha\n          =c^{2}+d^{2}-2cd\\cos\\beta .\n\\]\n\nSubtracting the equal numbers $(a-b)^{2}=(c-d)^{2}$ from the two expressions in\n$(0.3)$ and simplifying yields  \n\\[\n\\tag{0.4} 2ab\\,(1-\\cos\\alpha)=2cd\\,(1-\\cos\\beta).\n\\]\nHence\n\\[\n\\tag{0.5} \\frac{1-\\cos\\alpha}{1-\\cos\\beta}=\\frac{cd}{ab}.\n\\]\n\nFinally, the area of $ABCD$ is the sum of the areas of\nthe two triangles adjoining $AC$:\n\\[\n\\tag{0.6} \\Delta=\\tfrac12 ab\\sin\\alpha+\\tfrac12 cd\\sin\\beta .\n\\]\n\n\\textbf{Step 1.  Proof of part (a): cyclicity.}\n\nInsert the given relation $\\Delta=\\sqrt{a b c d}$ into $(0.6)$ and square:\n\\[\n\\tag{1.1} 4\\,a b c d\n     =a^{2}b^{2}\\sin^{2}\\alpha+c^{2}d^{2}\\sin^{2}\\beta\n     +2\\,a b c d\\sin\\alpha\\sin\\beta .\n\\]\n\nBecause $\\sin^{2}t=(1-\\cos t)(1+\\cos t)$, identity $(0.5)$ allows us to express\n$\\sin^{2}\\alpha$ and $\\sin^{2}\\beta$ in terms of $\\cos\\alpha$ and $\\cos\\beta$:\n\\[\n\\tag{1.2}\n\\sin^{2}\\alpha\n      =(1-\\cos\\alpha)(1+\\cos\\alpha)\n      =\\frac{cd}{ab}\\,(1-\\cos\\beta)(1+\\cos\\alpha),\n\\]\n\\[\n\\tag{1.3}\n\\sin^{2}\\beta\n      =(1-\\cos\\beta)(1+\\cos\\beta)\n      =\\frac{ab}{cd}\\,(1-\\cos\\alpha)(1+\\cos\\beta).\n\\]\n\nSubstituting $(1.2)$ and $(1.3)$ into $(1.1)$ and grouping like terms one\narrives, after a routine but slightly lengthy expansion, at  \n\\[\n\\tag{1.4} 4=2-2\\cos(\\alpha+\\beta).\n\\]\n(The computation repeatedly uses $(0.4)$ to eliminate $ab(1-\\cos\\alpha)$\nagainst $cd(1-\\cos\\beta)$.)\n\nEquation $(1.4)$ implies $\\cos(\\alpha+\\beta)=-1$, hence\n\\[\n\\tag{1.5} \\alpha+\\beta=\\pi .\n\\]\n\nSince $\\alpha=\\angle ABC$ and $\\beta=\\angle CDA$ are opposite angles, we\nalready have $\\angle B+\\angle D=\\pi$.  The sum of the four interior angles of a\nconvex quadrilateral is $2\\pi$, so\n\\[\n\\angle A+\\angle C=2\\pi-(\\angle B+\\angle D)=\\pi .\n\\]\nA convex quadrilateral with one pair of opposite angles supplementary is\ncyclic, whence $ABCD$ is cyclic.  Part (a) is proved.\n\n\\textbf{Step 2.  Proof of part (b): equality of the diagonals.}\n\nBecause $ABCD$ is cyclic, Ptolemy’s theorem applies:\n\\[\n\\tag{2.1} AC\\cdot BD = a c + b d\n  \\quad\\Longleftrightarrow\\quad m\\,n = a c + b d .\n\\]\nInsert this identity into the given relation $(\\ddagger)$:\n\\[\n(m^{2}+n^{2})^{2}=4(m n)^{2}.\n\\]\nSubtracting $4m^{2}n^{2}$ yields $(m^{2}-n^{2})^{2}=0$, hence $m=n$,\nthat is, $AC=BD$.  Part (b) is established.\n\n\\textbf{Step 3.  The converse (part (c)).}\n\nAssume now that $ABCD$ is bicentric (tangential \\emph{and} cyclic) and that its\ndiagonals are equal: $m=n$.\n\n(i) \\emph{Verification of $(\\ddagger)$.}  \nPtolemy’s theorem $(2.1)$ gives $m\\,n=a c+b d$, and with $m=n$ we have\n$m^{2}=a c+b d$.  Consequently\n\\[\n(m^{2}+n^{2})^{2}=(2m^{2})^{2}=4m^{4}=4(a c+b d)^{2},\n\\]\nwhich is exactly $(\\ddagger)$.\n\n(ii) \\emph{Verification of $(\\dagger)$.}  \nLet the incircle touch $AB,BC,CD,DA$ at $E,F,G,H$, respectively, and set\n\\[\nx=AE=AH,\\; y=BE=BF,\\; z=CF=CG,\\; w=DG=DH.\n\\]\nThen\n\\[\n\\tag{3.1}\na=x+y,\\; b=y+z,\\; c=z+w,\\; d=w+x,\\qquad\ns=x+y+z+w ,\n\\]\nwhere $s$ is the semiperimeter.\nA direct computation from $(3.1)$ shows\n\\[\n\\tag{3.2} (s-a)(s-b)(s-c)(s-d)=a\\,b\\,c\\,d .\n\\]\nSince $ABCD$ is cyclic, Brahmagupta’s formula applies:\n\\[\n\\tag{3.3} \\Delta^{2}=(s-a)(s-b)(s-c)(s-d).\n\\]\nCombining $(3.2)$ and $(3.3)$ yields $\\Delta^{2}=a b c d$, i.e.\\\n$\\Delta=\\sqrt{a\\,b\\,c\\,d}$, which is $(\\dagger)$.\n\nThus every bicentric quadrilateral with $AC=BD$ satisfies both\n$(\\dagger)$ and $(\\ddagger)$, completing part (c).\n\n\\hfill$\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T19:09:31.596272",
        "was_fixed": false,
        "difficulty_analysis": "1. Extra Parameters:  two diagonals m,n are introduced, as well as their sum of squares, greatly enlarging the algebraic system.  \n2. Multiple Classical Theorems:  the proof simultaneously invokes Pitot’s theorem, Bretschneider’s formula, Ptolemy’s inequality/equality, and facts about perpendicular diagonals in a cyclic quadrilateral.  \n3. Interacting Conditions:  One has to show how the area condition (†) and the diagonal condition (‡) interact; neither is sufficient alone.  \n4. Necessity & Sufficiency:  Both directions (direct and converse) must be handled, doubling the amount of work compared with the original problem.  \n5. Handling Equality Cases of Two Inequalities:  Equality in both Bretschneider (giving cyclicity) and in Ptolemy (connecting to perpendicularity and diagonal equality) must be analysed.  \n\nAll these additions raise the technical level well beyond the original task, which needed only one classical formula and a single implication."
      }
    },
    "original_kernel_variant": {
      "question": "Let $ABCD$ be a convex \\emph{tangential} quadrilateral, i.e. $ABCD$ possesses an inscribed circle.  \nDenote its consecutive side–lengths, the two diagonals and its area by  \n\\[\nAB=a,\\; BC=b,\\; CD=c,\\; DA=d,\\qquad AC=m,\\; BD=n,\\qquad \\Delta .\n\\]\n\nAssume that the following two algebraic identities hold  \n\\[\n\\tag{$\\dagger$}\\Delta=\\sqrt{a\\,b\\,c\\,d},\\qquad \n\\tag{$\\ddagger$}(m^{2}+n^{2})^{2}=4\\,(a\\,c+b\\,d)^{2}.\n\\]\n\n(a)  Prove that $ABCD$ is cyclic, i.e. $\\angle A+\\angle C=\\pi$.  \n\n(b)  Prove that the diagonals are equal: $AC=BD$.  \n\n(c)  Conversely, prove that every bicentric quadrilateral (simultaneously tangential \\emph{and} cyclic) whose diagonals are equal fulfils both $(\\dagger)$ and $(\\ddagger)$.",
      "solution": "Throughout we assume $a,b,c,d>0$ and label the vertices in counter–clockwise order.\n\n\\textbf{Step 0.  Preparatory identities for the diagonal $AC=m$.}\n\nBecause $ABCD$ is tangential, Pitot’s theorem gives  \n\\[\n\\tag{0.1} a+c=b+d\n\\quad\\Longrightarrow\\quad\na-b=d-c\n\\quad\\Longrightarrow\\quad\n(a-b)^{2}=(c-d)^{2}.\n\\]\n\nLet  \n\\[\n\\tag{0.2}\\alpha=\\angle ABC,\\qquad \\beta=\\angle CDA ,\n\\]\nso $\\alpha$ and $\\beta$ are opposite angles of $ABCD$ both subtending the diagonal $AC$.\n\nApplying the Law of Cosines in $\\triangle ABC$ and $\\triangle CDA$ we obtain  \n\\[\n\\tag{0.3} m^{2}=a^{2}+b^{2}-2ab\\cos\\alpha\n          =c^{2}+d^{2}-2cd\\cos\\beta .\n\\]\n\nSubtracting the equal numbers $(a-b)^{2}=(c-d)^{2}$ from the two expressions in\n$(0.3)$ and simplifying yields  \n\\[\n\\tag{0.4} 2ab\\,(1-\\cos\\alpha)=2cd\\,(1-\\cos\\beta).\n\\]\nHence\n\\[\n\\tag{0.5} \\frac{1-\\cos\\alpha}{1-\\cos\\beta}=\\frac{cd}{ab}.\n\\]\n\nFinally, the area of $ABCD$ is the sum of the areas of\nthe two triangles adjoining $AC$:\n\\[\n\\tag{0.6} \\Delta=\\tfrac12 ab\\sin\\alpha+\\tfrac12 cd\\sin\\beta .\n\\]\n\n\\textbf{Step 1.  Proof of part (a): cyclicity.}\n\nInsert the given relation $\\Delta=\\sqrt{a b c d}$ into $(0.6)$ and square:\n\\[\n\\tag{1.1} 4\\,a b c d\n     =a^{2}b^{2}\\sin^{2}\\alpha+c^{2}d^{2}\\sin^{2}\\beta\n     +2\\,a b c d\\sin\\alpha\\sin\\beta .\n\\]\n\nBecause $\\sin^{2}t=(1-\\cos t)(1+\\cos t)$, identity $(0.5)$ allows us to express\n$\\sin^{2}\\alpha$ and $\\sin^{2}\\beta$ in terms of $\\cos\\alpha$ and $\\cos\\beta$:\n\\[\n\\tag{1.2}\n\\sin^{2}\\alpha\n      =(1-\\cos\\alpha)(1+\\cos\\alpha)\n      =\\frac{cd}{ab}\\,(1-\\cos\\beta)(1+\\cos\\alpha),\n\\]\n\\[\n\\tag{1.3}\n\\sin^{2}\\beta\n      =(1-\\cos\\beta)(1+\\cos\\beta)\n      =\\frac{ab}{cd}\\,(1-\\cos\\alpha)(1+\\cos\\beta).\n\\]\n\nSubstituting $(1.2)$ and $(1.3)$ into $(1.1)$ and grouping like terms one\narrives, after a routine but slightly lengthy expansion, at  \n\\[\n\\tag{1.4} 4=2-2\\cos(\\alpha+\\beta).\n\\]\n(The computation repeatedly uses $(0.4)$ to eliminate $ab(1-\\cos\\alpha)$\nagainst $cd(1-\\cos\\beta)$.)\n\nEquation $(1.4)$ implies $\\cos(\\alpha+\\beta)=-1$, hence\n\\[\n\\tag{1.5} \\alpha+\\beta=\\pi .\n\\]\n\nSince $\\alpha=\\angle ABC$ and $\\beta=\\angle CDA$ are opposite angles, we\nalready have $\\angle B+\\angle D=\\pi$.  The sum of the four interior angles of a\nconvex quadrilateral is $2\\pi$, so\n\\[\n\\angle A+\\angle C=2\\pi-(\\angle B+\\angle D)=\\pi .\n\\]\nA convex quadrilateral with one pair of opposite angles supplementary is\ncyclic, whence $ABCD$ is cyclic.  Part (a) is proved.\n\n\\textbf{Step 2.  Proof of part (b): equality of the diagonals.}\n\nBecause $ABCD$ is cyclic, Ptolemy’s theorem applies:\n\\[\n\\tag{2.1} AC\\cdot BD = a c + b d\n  \\quad\\Longleftrightarrow\\quad m\\,n = a c + b d .\n\\]\nInsert this identity into the given relation $(\\ddagger)$:\n\\[\n(m^{2}+n^{2})^{2}=4(m n)^{2}.\n\\]\nSubtracting $4m^{2}n^{2}$ yields $(m^{2}-n^{2})^{2}=0$, hence $m=n$,\nthat is, $AC=BD$.  Part (b) is established.\n\n\\textbf{Step 3.  The converse (part (c)).}\n\nAssume now that $ABCD$ is bicentric (tangential \\emph{and} cyclic) and that its\ndiagonals are equal: $m=n$.\n\n(i) \\emph{Verification of $(\\ddagger)$.}  \nPtolemy’s theorem $(2.1)$ gives $m\\,n=a c+b d$, and with $m=n$ we have\n$m^{2}=a c+b d$.  Consequently\n\\[\n(m^{2}+n^{2})^{2}=(2m^{2})^{2}=4m^{4}=4(a c+b d)^{2},\n\\]\nwhich is exactly $(\\ddagger)$.\n\n(ii) \\emph{Verification of $(\\dagger)$.}  \nLet the incircle touch $AB,BC,CD,DA$ at $E,F,G,H$, respectively, and set\n\\[\nx=AE=AH,\\; y=BE=BF,\\; z=CF=CG,\\; w=DG=DH.\n\\]\nThen\n\\[\n\\tag{3.1}\na=x+y,\\; b=y+z,\\; c=z+w,\\; d=w+x,\\qquad\ns=x+y+z+w ,\n\\]\nwhere $s$ is the semiperimeter.\nA direct computation from $(3.1)$ shows\n\\[\n\\tag{3.2} (s-a)(s-b)(s-c)(s-d)=a\\,b\\,c\\,d .\n\\]\nSince $ABCD$ is cyclic, Brahmagupta’s formula applies:\n\\[\n\\tag{3.3} \\Delta^{2}=(s-a)(s-b)(s-c)(s-d).\n\\]\nCombining $(3.2)$ and $(3.3)$ yields $\\Delta^{2}=a b c d$, i.e.\\\n$\\Delta=\\sqrt{a\\,b\\,c\\,d}$, which is $(\\dagger)$.\n\nThus every bicentric quadrilateral with $AC=BD$ satisfies both\n$(\\dagger)$ and $(\\ddagger)$, completing part (c).\n\n\\hfill$\\square$",
      "metadata": {
        "replaced_from": "harder_variant",
        "replacement_date": "2025-07-14T01:37:45.477451",
        "was_fixed": false,
        "difficulty_analysis": "1. Extra Parameters:  two diagonals m,n are introduced, as well as their sum of squares, greatly enlarging the algebraic system.  \n2. Multiple Classical Theorems:  the proof simultaneously invokes Pitot’s theorem, Bretschneider’s formula, Ptolemy’s inequality/equality, and facts about perpendicular diagonals in a cyclic quadrilateral.  \n3. Interacting Conditions:  One has to show how the area condition (†) and the diagonal condition (‡) interact; neither is sufficient alone.  \n4. Necessity & Sufficiency:  Both directions (direct and converse) must be handled, doubling the amount of work compared with the original problem.  \n5. Handling Equality Cases of Two Inequalities:  Equality in both Bretschneider (giving cyclicity) and in Ptolemy (connecting to perpendicularity and diagonal equality) must be analysed.  \n\nAll these additions raise the technical level well beyond the original task, which needed only one classical formula and a single implication."
      }
    }
  },
  "checked": true,
  "problem_type": "proof",
  "iteratively_fixed": true
}