Question

Let ABCD be a trapezoid with bases AB and CD. Then
2 2 2
2 2 2
A B B C A C
C D A D A C
− +
=

Answer 1

$\huge \boxed{ \frac{ {AB}^{2} - {AD}^{2} + {BD}^{2} }{ {CD}^{2} - {BC}^{2} + BD {}^{2} } }$