Question

If the angle of vertex of an isosceles triangle is 100∘, then the measure of a base angle equals .

• A. ${40}^{\circ }$
• B. ${80}^{\circ }$
• C. ${20}^{\circ }$

Answer 1

The correct option is A ${40}^{\circ }$


Given that $\angle A={100}^{\circ }$
Since $△ABC$ is isosceles, we have $\angle ABC=\angle ACB=x\left(say\right)$
Now, $\angle x+\angle x+{100}^{\circ }={180}^{\circ }\left(\text{Using angle sum property}\right)$
$⟹2x+{100}^{\circ }={180}^{\circ }$
$⟹2x={80}^{\circ }⟹x={40}^{\circ }$