ABCD is a cyclic quadrilateral. If angle DAB = 78° and angle ABC = 105°, find angle BCD.
A75°
B95°
C102°CORRECT
D115°
GCEMathematicsPlane Geometry2021
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Why the answer is C, and why the others tempt you.
Opposite angles of a cyclic quadrilateral are supplementary. Therefore angle BCD = 180° − angle DAB = 180° − 78° = 102°. Similarly, angle ADC = 180° − 105° = 75°.
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