Question

In a $\Delta ABC,ifAB=ACand\angle B={70}^{\circ },find\angle A.$

Answer 1

In $\Delta ABC,\angle B={70}^{\circ }$

AB = AC

$\therefore \angle B=\angle C$

(Angles opposite to equal sides)

But $\angle B={70}^{\circ }$

$\therefore \angle C={70}^{\circ }$

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

(Sum of angles of a triangle)

$\Rightarrow \angle A+{70}^{\circ }+{70}^{\circ }={180}^{\circ }$

$\Rightarrow \angle A+{140}^{\circ }={180}^{\circ }$

$\therefore \angle A={180}^{\circ }-{140}^{\circ }={40}^{\circ }$