Answer
1. The length of a diagonal of a parallelogram is directly proportional to the size of the angles opposite the diagonal.
2. Thus, the only way for the diagonals to be equal is if the angles are equal.
3. Since the angles add to 360 degrees, they will only be equal if they are 90 degrees.
4. If the angles are 90 degrees, then it is a rectangle.
5. Thus, if the diagonals are equal, it is a rectangle.
