Class 9 | Chapter 10 | Circles | Theorem 10.12

Class 9 - Theorem 10.7

Theorem 10.12: If the sum of a pair of opposite angles of a quadrilateral is 180 degree, the quadrilateral is cyclic.

Prerequisite

This is relatively bit tricky theorem to prove (not difficult though) in this chapter. There are two prerequisite theorems that you must know before you solve this theorem:

Theorem 10.5 – There is (always) one and only one circle passing through three given non-collinear points.

 

Theorem 10.11 – The sum of either pair of opposite angles of a cyclic quadrilateral is 180 degrees.

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