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If the bisectors of the base angles of a triangle enclose an angle of $135^o$. Prove that the triangle is a right triangle.
Given:
The bisectors of the base angles of a triangle enclose an angle of $135^o$.
To do:
We have to prove that the triangle is a right triangle.
Solution:
Let in $\triangle ABC, OB$ and $OC$ be the bisectors of $\angle B$ and $\angle C$ and $\angle BOC = 135^o$
$\angle A+\angle B+\angle C=180^{\circ}$
Dividing both sides by 2, we get,
$\frac{1}{2} \angle A+\frac{1}{2} \angle B+\frac{1}{2} \angle C=180^{\circ}$
$\frac{1}{2} \angle A+\angle O B C+\angle O B C=90^{\circ}$
$\angle O B C+\angle O C B=90^{\circ}-\frac{1}{2}A$
In $\triangle B O C$,
$\angle B O C+\angle O B C+\angle O C B=180^{\circ}$
$\angle B O C+90^{\circ}-\frac{1}{2} \angle A=180^{\circ}$
$\angle B O C=90^{\circ}+\frac{1}{2} \angle A$
This implies,
$90^o+ \frac{1}{2}\angle A = 135^o$
$\frac{1}{2}\angle A= 135^o -90^o = 45^o$
Therefore,
$\angle A = 2(45^o) = 90^o$
Hence, $\triangle ABC$ is a right-angled triangle.