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Videos, worksheets, games and activities to help Geometry students learn about angles in a semi-circles and cyclic quadrilaterals.

Angles in Semicircle :

If an angle is inscribed in a semicircle, it will be half the measure of a semicircle (180 degrees), therefore measuring 90 degrees. Angles in semicircle is one way of finding missing missing angles and lengths. Pythagorean theorem can be used to find missing lengths (remember that the diameter is the hypotenuse). Also, the measure of an angle formed by a chord to a tangent is half the intercepted arc. Angles in a semi-circle are 90 degrees This circle theorem can be looked at in two ways, either as 'angle at the center is twice the angle at circumference' or 'angles in a semi-circle are 90 degrees'

A cyclic quadrilateral has vertices on the same circle and is inscribed in the circle. The opposite angles have the same endpoints (the other vertices) and together their intercepted arcs include the entire circle. Since the measure of an inscribed angle is half the intercepted arc, the sum of the opposite angles must be 180 degrees. Proof Cyclic Quadrilaterals

This video explains why the opposite angles in a cyclic quadrilateral add up to 180 degrees.

How to use circle properties to find missing sides and angles

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