Q: 2 Prove that if chords of congruent circles subtend equal angles at their centres, then the chords are equal.
Given : chords of congruent circles subtend equal angles at their centres,
To prove: $BC = QR$
Proof:
In $\triangle X Y Z$ and $\triangle P Q R$,
$\angle \mathrm{YXZ}=\angle \mathrm{QPR}$ (Given)
$X Y=P Q$ (Radii of congruent circle)
$\mathrm{XZ}=\mathrm{PR} \quad$ (Radii of congruent circle)
Thus, $\triangle X Y Z \cong \triangle P Q R($ By SAS rule)
$\mathrm{YZ}=\mathrm{QR}$ (CPCT)