Answer
Consider a rectangle with its longest side being horizontal.
1. The rhombus will form four triangles, each in a corner of the rectangle.
2. By the definition of midpoint, the two sides next to the 90 degree angles for the all of the triangles will be congruent.
3. By SAS, this means that the triangles are congruent.
4. CPCTC, so each of the lengths of the rhombus formed are congruent.
5. Since sides opposite to each other are congruent, it is a parallelogram.
6. Since adjacent sides are congruent and it is a parallelogram, it is a rhombus.
