Problem with vectors ?

1. Mar 31, 2010

lorik

Ok
Heres the text :Find the angle of vectors a=3p+2q and b=p+5q and if p and q are orthogonal unit ? ... ?
ok the formula is pretty simple cos=a*b/!a! * !b! =
But once i progress and I get 3p square + 15pq + 2qp +10q square ,now I see here that 15pq and 2qp are eliminated is just BEEEYOOOND MEEE I MEAN hows that possible ,thanks in advance ?!?!?

2. Mar 31, 2010

mathman

p and q are orthogonal, which means (definition) that their dot product is 0.

3. Mar 31, 2010

lorik

More specific please Im trying to understand for future references ,thnx

4. Mar 31, 2010

Fragment

Have you tried finding the vector sum of your two vectors? As the vector sum can be plugged into your formula, along with the two original vectors to find your angle, in which case you will need to do the inverse cos function. It.

theta=cos^-1((|A|*|B|)/AB))

Where * is dotted on.

Good luck.

5. Apr 1, 2010

mathman

Let a and b be arbitrary vectors (any dimension). The definition of orthogonal is the dot product is 0. Question for you - do you know what a dot product is?

6. Apr 1, 2010

lorik

dot product !a! * !b! *cos theta

But how in general can orthogonal plane have a dot product of 0 ?

7. Apr 1, 2010

rock.freak667

If they are orthogonal, what is the angle between the vectors?

8. Apr 1, 2010

lorik

like Zero ,lol ?

9. Apr 1, 2010

Staff: Mentor

Like, no.
Do you know what orthogonal means?

What you're asking here makes no sense.