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

3. 80°

Solution:

In $\triangle\text{ABC,}$

$\text{AB = AC}$

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

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

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

$\Rightarrow\angle\text{A}+50^{\circ}+50^{\circ}=180^{\circ}$

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