In the given figure, if ∠AOC=130∘, find the value of ∠ABC .
Given ∠AOC=130∘.
Consider the arc ABC. Consider a point P not on this arc, but on the circumference of the circle. Join AP and CP.
Arc ABC subtends ∠AOC at centre and ∠APC on the remaining part of the circle (at the point P).
Since the angle subtended by an arc at the centre is double the angle subtended by it on any remaining point of the circle, we have
∠APC=12∠AOC.
⟹∠APC=12×130∘=65∘
Since APCB is a cylic quadrilateral, we have its opposite angles to be supplementary.
⟹∠ABC+∠APC=180∘
⟹∠ABC+65∘=180∘
⟹∠ABC=180∘−65∘=115∘