Congruence of Triangles and Inequalities in a Triangle — Maths STD 9 — Question

1. 50°

Solution:

In $\triangle\text{ABC,}$

$\text{BC = AB}$

$\Rightarrow\angle\text{A}=\angle\text{C}$ (angles opposite to equal sides are equal)

Now, $\angle\text{A}+\angle\text{B}+\angle\text{C}=180^{\circ}$

$\Rightarrow\angle\text{A}+80^{\circ}+\angle\text{A}=180^{\circ}$

$\Rightarrow2\angle\text{A}+100^{\circ}$

$\Rightarrow\angle\text{A}=50^{\circ}$