Angles equation for geometry.

40 degrees

Step-by-step explanation:

Since we have the diameter included in the triangle, the angle at B is 90

This is because angle at center is two times the angle at the circumference

Since we have the diameter, the angle at the center will be 180 degrees so the angle A at the circumference will be 90 degrees

Now, let us use the measure of the arc given

The angle at C is half of 100 which makes it 50

This is according to an angle arc relationship

Lastly, the sum of angles in a triangle is 180 degrees

We already have two angles and we need the last

so we have to add what we have already and subtract from 180 to get the angle we are looking for

So we have the angle as 180-(90 + 50) = 180 -140 = 40