Cyclic quadrilaterals

A cyclic quadrilateral is a quadrilateral drawn inside a circle so that its corners lie on the circumference of the circle. You should know that:

(a) the opposite angles of a cyclic quadrilateral sum to 180°
i.e. a+ c = 180°
b + d = 180°

(b) the exterior angle of a cyclic quadrilateral is equal to the interior
opposite angle
i.e. e = c

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