Prove that a parallelogram with 1 angle equal to 90 ° is a rectangle.

Let’s complete the task:
A parallelogram is a quadrilateral in which opposite sides are equal and parallel and opposite angles are equal. From the condition of the problem, it is known that one angle of the parallelogram is 90 degrees, which means that the opposite angle is also 90 degrees. The sum of all four angles of a parallelogram is 360 degrees.
The other two angles will be 360 ​​- 180 = 180 degrees, and each of them will be 180/2 = 90 degrees. If a parallelogram has one angle of 90 degrees, then the other three angles are 90 degrees each, and such a quadrangle is a rectangle.