Question

In $\Delta$ABC, side AB$=$AC, $\angle$B$={70}^o$. Find m$\angle$A and m$\angle$C.

Answer 1

$\because AB=AC$

$\therefore \angle B=\angle C$

[angles opposite to equal sides of a $\Delta$ are equal]

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

In $\Delta ABC$

$\angle A+\angle B+\angle C={180}^{\circ }$ (angle sum prop.)

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

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

$\Rightarrow \angle A={40}^{\circ }$