Given : A circle with centre P, XY and YZ are two chords.
To prove : ∠XPZ=2(∠XZY+∠YXZ)
Proof : In a circle with centre P, arc XY subtends ∠XPY at the centre and ∠XZY at remaining part of the circle.
⇒∠XPY=2∠XZY ....(1)
Similarly, arc YZ subtends ∠YPZ at the centre and ∠YXZ at remaining part
∴∠YPZ=2∠YXZ....(2)
Adding (1), and (2), we get
∠XPY+∠YPZ=2∠XZY+2∠YXZ
⇒∠XPZ=2(∠XZY+∠YXZ).