Question

Find the measure of an exterior angle at the base of an isosceles triangle measures ${78}^o$

• A. ${100}^o$
• B. ${101}^o$
• C. ${102}^o$
• D. ${103}^o$

Answer 1

The correct option is C ${102}^o$

Base angles of an isosceles triangle are equal

$\therefore \angle B=\angle C={78}^{\circ }$

Now $x+\angle C={180}^{\circ }$ (Angles made on straight line)

$x={180}^{\circ }-{78}^{\circ }={102}^{\circ }$

So exterior angle is ${102}^{\circ }$