The ABC triangle is rectangular. Calculate angle A. If angle B is 40 degrees and angle C is unknown.

We need to find one of the corners of the triangle.

Let us apply the theorem on the sum of the angles of a triangle to solve. It says that the sum of the angles of a triangle is 180 °.

The condition says that triangle ABC is rectangular (that is, it has an angle equal to 90 ° and most likely this is angle C). We need to find the degree measure of the angle A. And we also know the degree measure of the angle B 40 °.

We denote by x ° the degree measure of the unknown angle and we get the equation:

90 + 40 + x = 180;

130 + x = 180;

x = 180 – 130;

x = 50 ° angle B.