# Dot Product help

1. Jul 28, 2006

### Hollysmoke

-Use the dot product to prove that the quadrilateral with vertices A(-4,1) B(4,5) C(7,-1) and D(-1,-5) is a rectangle.

I tried find AB dot BC = 0, but I'm not getting 0. So I'm stumped. Can someone help me out? =/

2. Jul 28, 2006

### 0rthodontist

3. Jul 28, 2006

### end3r7

Try BA dot BC

4. Jul 28, 2006

### HallsofIvy

Staff Emeritus
You are aware, are you not, that AB= -BA so that if AB dot BC= 0, so is BA dot BC?

Hollysmoke, could you show us what you got for AB and BC? I wondering if you were sufficiently careful of signs while subtracting.

5. Jul 28, 2006

### end3r7

pfft, you are right. That's what I get for posting late at night.