**Data:** Two circles having centres A and B, intersect at C and D.

**To Prove:** ∠ACB = ∠ADB.

**Construction:** Join A and B.

**Proof:**

In ∆ABC and ∆ABD,

AC = AD (∵ radii of same circle are equal)

BC = DD

AB is common.

∴ ∆ABC ≅ ∆ABD (SSS Postulate.)

∴ ∠ACB = ∠ADB.