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.
Write out the three known sides. For example, assume you have angles of 120, 120 and 60.
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.
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Solve for X. Add together all of the angles. Subtract the amount from 360. In the example, 60 degrees is the missing angle.