The Lesson

A cyclic quadrilateral is a 4 sided shape where each corner touches the circle. Both pairs of opposite angles add up to 180°.

Lesson Slides

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How to Use the Circle Theorem where Opposite Angles in a Cyclic Quadrilateral Add Up to 180°


What is the angle θ in the circle below?



The opposite angles add up to 180°.
70° + θ = 180°


Subtract the known angle from 180°.
θ = 180° − 70° = 110°


The angle θ is 110°.