To show that the parallelogram's opposite angles are equal.
Create a parallelogram ABCD by drawing a parallelogram ABCD. Join the BD team.
Proof: ABD and BCD form a triangle.
[opposite sides of the parallelogram] AB = CD
[opposite sides of the parallelogram] BC = DA
BD is a very frequent condition.
As a result, triangles ABD and BCD are identical.
As a result, A = C, with sides opposing BD.
In the same way, B = D.
As a result, the parallelogram's opposite angles are equal.
I hope this helps, even if it's just a little :)