Rhombus Diagonals

Use coordinate geometry to prove that the diagonals of a rhombus are perpendicular. That is, write a coordinate geometry proof that formally proves what this applet informally illustrates. Be sure to assign appropriate variable coordinates to your rhombus's vertices! (Believe it or not, the maximum number of variable coordinates you'll actually need is 2.) (You can use 3 variable coordinates if you're willing to work with a system of equations.)
Now prove this same theorem true using the format of a 2-column proof or a paragraph proof.