Isosceles Trapezoid Midpoints

Topic:
Trapezoid
Use coordinate geometry to prove that the midpoints of the sides of an isosceles trapezoid are vertices of a rhombus. That is, write a coordinate geometry proof that formally proves what this applet informally illustrates. Be sure to assign appropriate variable coordinates to your isosceles trapezoid's vertices! (The maximum number of variable coordinates your coordinate setup should have is 3.)
Now prove this same theorem true using the format of a 2-column proof or a paragraph proof. Hint: This proof may contain multiple pairs of congruent triangles.