One theorem states that if one angle of a parallelogram is a right angle, then the parallelogram is a rectangle. I did the following two column proof below to show this. Is it correct?
Suppose we have parallelogram ABCD where A = lower left vertex, B = upper left vertex, C = upper right vertex, and D = lower right vertex. Suppose A is the right angle given.
1. ABCD is a parallelogram/Given
2. m∠A = 90°/Given
3. m∠A ≅ m∠C/Definition of parallelogram
4. m∠A + m∠B = 180/Same side interior angles are supplementary.
5. m∠A ≅ m∠B/Subtraction property of equality
6. m∠B ≅ m∠D/Definition of parallelogram
7. ABCD is a rectangle/Definition of rectangle
The one I'm really worrying about is #5. Can I use subtraction property of equality to justify that? Or do I just use the given?