Hmmmm. Not sure about this.

Question

anonymous · Answer

attachment

phi · Answer

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

anonymous · Answer

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

phi · Answer

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

phi · Answer

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

phi · Answer

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.

anonymous · Answer

Okay got it.