# Yet another vector proof

1. Oct 20, 2012

### Bipolarity

1. The problem statement, all variables and given/known data
If $(a \times b) \times (c \times d) = 0$, show that a,b,c,d are coplanar.

2. Relevant equations

3. The attempt at a solution
I have done the converse of this problem, but am having trouble on how to do this. Can we perhaps use Lagrange's identity. How can I start?

BiP

2. Oct 21, 2012

### SammyS

Staff Emeritus
It's not true.

If $(a \times b)=0\,,$ the c & d can be anything.

3. Oct 21, 2012

### Bipolarity

Thanks Sammy!

BiP