Originally Posted by

**AshleyT** Ok,

When you are finding an angle between two vectors, does it matter what direction their going in? I didn't think it did did...i mean an angle between two lines is an angle.

So this was the question.

So need to use formula a.b = /a//b/Cos C (/a/ means for modulus, not dividing)

Where AB = a and BC = b

So i worked out

AB as (1, -3, -3)

and

BC as (-2, 1, -2)

a.b = -2 -3 + 6 = 1

/a/ = root 19

/b/ = 3

$\displaystyle cos c = \frac{1}{3\sqrt{19}}$

and therefore c = 86 degrees.

Well it was wrong and the answer was 94 degrees...and in the answer book it said i needed to do vector CB to get

$\displaystyle cos c = \frac{-1}{3\sqrt{19}}$

My question is why? Why does it matter if i use BC or CB and more importantly how will i know in an exam which one to use? No diagram was given...=(

Thanks!