Q: 8 If the non-parallel sides of a trapezium are equal, prove that it is cyclic.
Given: $ABCD$ is a trapezium.
Construction: Draw $AD || BE$.
Proof: In quadrilateral $ABED$,
$AB || DE$ (Given)
$AD || BE$ (By construction)
Thus, $ABED$ is a parallelogram.
$AD = BE$ (Opposite sides of the parallelogram )
$AD = BC$ (Given)
So, $BE = BC$
In $\triangle EBC$,
$BE = BC$ (Proved above)
Thus, $\angle C=\angle 2$-----1(angles opposite to equal sides)
$\angle A=\angle 1$-------2(Opposite angles of the parallelogram)
From 1 and 2 , we get
$\angle 1+\angle 2=180^{\circ}$
$\Rightarrow \angle A+\angle C=180^{\circ}$
Thus, $ABED$ is a cyclic quadrilateral.