@wizengy

This sounds incomplete. Opposite equal sides of a rectangle can also make a parallelogram with non-right angles. I will give you that opposite angles are the same but you need to show the one additional step to prove just one right angles.
I also know this is a carpenter's technique to find the center of a circle by taking a square and putting one corner on the circle to find the diameter. Repeat 90 degrees or so around, and it shows the center where the lines cross.

@navyachoraria

In school, we just proved this by proving that for any two points on the circumference on the circle, the angle they make at the center will be twice of the angle they make at any other point on the circle

@Athashag

i dont know what this thing is