Question

The picture is not drawn to scale. Find the value of B. Click Here for Help Video

129°
A°
B

Answer 1

39°

Explanation:

We can solve this using the exterior angle theorem

Exterior angle theorem:

The measure of an exterior angle of a triangle , is equal to the sum of the two remote interior angles (see attached image.)

Here, we are given the exterior angle as well as one of the interior angles (the right angle.)

Given:

Exterior angle = 129°

Interior angle 1 = 90°

Interior angle 2 = B

Using the exterior angle theorem,

exterior angle = interior angle 1 + interior angle 2

so

• 129 = 90 + B

subtract 90 from both sides

• B = 39°

The measure of B is 39 degrees