- If angle A is supplementary to angle B, then angle A=180-angle B
- If angle B is supplementary to angle C, then angle C=180-angle B
- For the first 2 arguments we have angle A=angle C
-If angle C is supplementary to angle D, then angle D=180-angle C
- If angle B is supplementary to angle C, then angle B=180-angle C
- For the last two arguments we have angle B=angle D
If angle A=angle C and angle B=angle D the quadrilateral ABCD is a parallelogram
