In the given figure, O is the centre of a circle. If ∠OAB = 40°, then ∠ACB = ?
Given: ∠OAB = 40°
Consider ΔAOB
Here,
OA = OB (radius)
∠OBA = ∠OAB = 40° (angles opposite to equal sides are equal)
By angle sum property
∠OBA + ∠OAB + ∠AOB = 180°
40° + 40° + ∠AOB = 180°
∠AOB = 180° — 40° — 40° = 100°
We know that,
∠AOB = 2× ∠ACB
∠AOB = ∠ACB
×100° = ∠ACB
∠ACB = 50°
∴∠ACB = 50°