Question

In triangle ABC, if BC=AC and $\angle$ B =40${}^{\circ }$. Then $\angle$ C is equal to ___ degrees.

• A. 100^o
• B. 200
• C. 300
• D. 400

Answer 1

The correct option is A

100^o

In a triangle, if two sides are equal then it is an isosceles triangle in which the angles opposite to the two equal sides are also equal.

Therefore, $\angle A=\angle B$

Since sum of all the angles in a triangle $={180}^{\circ }$

$\Rightarrow$ $\angle A+\angle B+\angle C={180}^{\circ }$

$\Rightarrow$ $2\times {40}^{\circ }+\angle C={180}^{\circ }$ ($\angle A=\angle B$)

$\Rightarrow$ $\angle C$$={180}^{\circ }-2\times {40}^{\circ }$

$\Rightarrow$ $\angle C$$={100}^{\circ }$