Q: 1 Recall that two circles are congruent if they have the same radii. Prove that equal chords of congruent circles subtend equal angles at their centres.
Given: The two circles are congruent if they have the same radii.
To prove: The equal chords of congruent circles subtend equal angles at their centres i.e. QPR=YXZ
Proof :
In $\triangle XYZ$ and $\triangle PQR$,
$QR = YZ$ (Given)
$PQ = XY$ (Radii of congruent circle)
$PR = XZ$ (Radii of congruent circle)
Thus, $\triangle PQR \cong \triangle X Y Z$ (By SSS rule)
$\angle Q P R=\angle \mathrm{YXZ}$ (CPCT)