Question

Hmmmm. Not sure about this.

Answer 1

One definition of a parallelogram is 4 sides, whose opposite sides are =

Answer 2

I know that it is true. But I'm not sure how to write a proof for this.

Answer 3

Of course, if you know the opposite sides of a 4-sided figure are =, you can prove the opposite sides are parallel, which is the other definition of a parallelogram

Answer 4

It seems with these problems put on your "hunt for congruent triangles" hat
bisect suggests sides. crossing diagonals suggest = vertex angles.

Answer 5

Put a single hash mark on the 2 halves of one of the diagonals, and 2 hash marks on the halves of the other diagonal.
mark the = vertex angles. You are looking at a SAS congruent triangle.

Answer 6

Okay got it.