Write True or False and justify your answer in each of the following: In Fig. 10.10, if AOB is a diameter and ADC = 120°, then CAB = 30°.
True
Solution:
Given, AOB is a diameter and ADC = 120°
Join CA & CB
Since ADCB is a cyclic quadrilateral
ADC + CBA = 180° (Sum of opposite angles of a cyclic quadrilateral is 180°)
120° + CBA = 180°
CBA = 60°
In ACB,
CAB + CBA +ACB = 180° (angle sum property of a triangle)
ACB = 90° (Since, ACB is an angle in a semi-circle)
CAB + 60° + 90° = 180°
CAB = 180° – 150° = 30°
Therefore the given statement is true.