# Thread: why is this false?

1. ## why is this false?

Two vectors u, v are orthogonal if and only if ||u+v||^2 +||u-v||^2 = 2||u||^2 - 2||v||^2?

Doesn't (u+v)(u+v) + (u-v)(u-v) = u^2 + v^2?

2. edited for stupidity

3. Good question man, I don't see how orthogonality plays any role.
$||u+v||^2+ ||u-v||^2=(u+v)\cdot(u+v)+(u-v)\cdot(u-v)$
$||u||^2+u\cdot v + v\cdot u +||v||^2 + ||u||^2-u\cdot v - v\cdot u +||v||^2 = 2||u||^2+2||v||^2$