In the following figure, PQ = PR and SQ = SR, then
∠PRQ = ∠SRQ
∠PQS = ∠PQR
∠PQR = ∠SQR
∠PQS = ∠PRS
Join P and S. The angles opposite the equal sides of a triangle are equal.
The diagonal PR of a quadrilateral PQRS bisects the angles P and R, then