------
## 最终结论
```python
def median_trapezium(a, b, c):
    return (a + b) / 2

# Test cases
assert median_trapezium(15, 25, 35) == 20
assert median_trapezium(10, 20, 30) == 15
assert median_trapezium(6, 9, 4) == 7.5
```