http://img2.freeimagehosting.net/uploads/cba456ce42.jpg

Printable View

- Oct 1st 2007, 02:00 AMafeasfaerw23231233vectors problem #3
- Oct 1st 2007, 02:57 AMearboth
- Oct 1st 2007, 04:56 AMafeasfaerw23231233
well, but the answer in my textbooks says theta is 120 degree!!

how can i get 120 degree from this? - Oct 1st 2007, 05:53 AMtopsquark
u and v are unit vectors, so |u| = |v| = 1.

We are also told that u + v is a unit vector, so

$\displaystyle (u + v) \cdot (u + v) = |u + v|^2 = 1$

(Since for any vector x, $\displaystyle x \cdot x = x^2$.)

But

$\displaystyle (u + v) \cdot (u + v) = u^2 + v^2 + 2 u \cdot v$

What can you make of all this?

-Dan - Oct 1st 2007, 06:33 AMafeasfaerw23231233
oh, i know it now. thanks!