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If an angle is an inscribed angle, then its measure is equal to half the measure of the intercepted arc.

Substitution Property of Equality

Definition of a semicircle

Multiplication Property of Equality

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Given circle A, show that m∠CDB = 90°.
m
BC = 180°
m∠CDB = 1 over 2 1 2 m
BC
m∠CDB = 1 over 2 1 2 (180°)
m∠CDB = 90°
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