Let PR and PQ be two tangents to the circle with centre O and let RQ be a chord of the circle.
We have to prove that ∠PQR=∠PRQ
Now, PQ=PR(Since tangents drawn from an external point to a circle are equal)
In ΔPQR, ∠PQR=∠PRQ
(Since opposite sides are equal, their base angles are also equal)
Hence, proved.