Question

1. 在圆内接四边形abcd中，若∠adc = 70°，则∠abc =（）

a. 70° b. 80° c. 100° d. 110°

Explanation:

Step1: Recall cyclic quadrilateral property

Opposite angles sum to $180^\circ$.

Step2: Calculate ∠ABC

$\angle ABC = 180^\circ - \angle ADC = 180^\circ - 70^\circ = 110^\circ$

Answer:

D. 110°