# Vector multiplication

1. Aug 23, 2014

### Roodles01

1. The problem statement, all variables and given/known data
Still having a little trouble so here's the problem.

(ex + ez) x (3ey - 4ez)

3. The attempt at a solution

(ex * ez) + (ex * (-4ez)) + (ez * 3ey) + ( ez * (-4ez)

now, these are all orthogonal to each other, so, for example, if I have ex * ey then I should end up with ez, shouldn't I?
So here is my solution.

= 3ez - 4ey + 3ex - 4ez
= 3ex - 4 ey + ez

The solution shown to be correct is -3ex + 4ey + 3ez

so what have I done wrong, please?

2. Aug 23, 2014

### Staff: Mentor

I assume by * you mean X (vector product). So yes, $e_x \times e_y = e_z$. What about $e_y \times e_x$, $e_x \times e_z$, and the other combinations? (You are making a sign error.)

3. Aug 23, 2014

### Roodles01

Thank you.
A simple thing becomes clear again.