How To Calculate The Missing Angle Of A Rectangle

A rectangle is a geometric figure in which all four angles are 90 degrees. So if you have a missing side and it is a true rectangle, then you know the missing angle is 90 degrees. However, you could be working with a rectangle that is slanted. This is known as a parallelogram. To find the missing angle in such a case, you need to know that all the angles will add up to 360 degrees.

Step 1

Write out the three known sides. For example, assume you have angles of 120, 120 and 60.

Step 2

Write out an algebraic expression. Because you know the sum of all four sides must equal 360 degrees, your expression in the example will be 120 + 120 + 60 + X = 360.

Step 3

Solve for X. Add together all of the angles. Subtract the amount from 360. In the example, 60 degrees is the missing angle.