Two angles that have the sum of 180 degrees are supplementary angles. Given one supplementary angle A, a person can find the other supplementary angle B by subtracting A from 180 degrees.
If one of the supplementary angles is 40, then the other supplementary angle is 180 – 40, or 140 degrees. When added together, two supplementary angles also form a straight angle; thus, a straight angle is also equal to 180 degrees.
In geometry, there are other different types of angles besides supplementary angles, such as acute and obtuse angles. While an acute angle is less than 90 degrees, an obtuse angle is more than 90 degrees but less than 180 degrees. In the example given above, the 40-degree angle is an acute angle, and the 140-degree angle is an obtuse angle.