#### Answer

When the midpoints of a rectangle are joined, it forms a rhombus.

#### Work Step by Step

We assign coordinates for the rectangle:
$(0,0); (0,2a); (2b,2a); (2b,0)$
Thus, we find the length of each side:
$s_1 = \sqrt{b^2 + a^2}$
$s_2 = \sqrt{b^2 + a^2}$
$s_3 = \sqrt{b^2 + a^2}$
$s_4 = \sqrt{b^2 + a^2}$
The sides are equal lengths, so it is a rhombus.