$\"\"$

Observe the right triangle figure, the angle B is $\"\"$ in right triangle ABC.

The segment $\"\"$ is bisects the angle B ( $\"\"$ ).

So the angle B is $\"\"$ in triangle ADB.

The sum of the three angles in a triangle is $\"\"$ .

In triangle ADB, $\"\"$ .

The values of $\"\"$ are substitute in the $\"\"$ .

$\"\"$

Subtract $\"\"$ from each side.

$\"\"$

$\"\"$ .

$\"\"$

The value of $\"\"$ .