# Find theta from the cross product and dot product of two vectors

loganblacke

## Homework Statement

If the cross product of vector v cross vector w = 3i + j + 4k, and the dot product of vector v dot vector w = 4, and theta is the angle between vector v and vector w, find tan(theta) and theta.

## Homework Equations

vector c = |v||w| sin(theta) where vector c is the cross product of v and w.

## The Attempt at a Solution

I'm assuming you have to split the cross product back into the two original vectors and then calculate the angle but I'm not sure how to go from cross product to 2 vectors. Please help!

## Answers and Replies

Science Advisor
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You can't get the two vectors. And you don't have to.
|3i + j + 4k|=|v|*|w|*sin(theta). 4=|v|*|w|*cos(theta). How would you get tan(theta) from that?

loganblacke
You can't get the two vectors. And you don't have to.
|3i + j + 4k|=|v|*|w|*sin(theta). 4=|v|*|w|*cos(theta). How would you get tan(theta) from that?

I honestly have no idea.

Staff Emeritus
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Education Advisor
Think trig identity.

Science Advisor
Homework Helper
Think trig identity.

That's coy. :) What's the definition of tan(theta)?

loganblacke
That's coy. :) What's the definition of tan(theta)?

tan theta is sin theta/cos theta.. which I think would put the vector over its magnitude and result in tan theta = unit vector..

Science Advisor
Homework Helper
tan theta is sin theta/cos theta.. which I think would put the vector over its magnitude and result in tan theta = unit vector..

??? Divide the two sides of the equations by each other. Can't you find a way to get tan(theta) on one side?

loganblacke
??? Divide the two sides of the equations by each other. Can't you find a way to get tan(theta) on one side?

I'm completely lost right now, the only thing i can work out on paper is if you isolate |v|*|w| in both equations by dividing both sides by cos theta and sin theta respectively. Then you could set the vector/sin theta = 4/cos theta.

Science Advisor
Homework Helper
I'm completely lost right now, the only thing i can work out on paper is if you isolate |v|*|w| in both equations by dividing both sides by cos theta and sin theta respectively. Then you could set the vector/sin theta = 4/cos theta.

There aren't any vectors here anymore, there's only |3i + j + 4k|. That's number, not a vector. You can compute it. Can't you get sin(theta)/cos(theta) on one side and a number on the the other?

Last edited:
loganblacke
There aren't any vectors here anymore. Everything is just numbers. Sure isolate |v|*|w| in both equations. Then set the other sides equal to each other. What's the resulting equation?

I see now that its the magnitude of vector 3i + J + 4k rather than the vector itself. So you end up with sqrt(3^2+1^2+4^2)/sin theta = 4/cos theta..

So you end up with tan theta = sqrt(26)/4.

loganblacke
then theta = arctan(sqrt(26)/4)

Thanks for the help.. again.

Staff Emeritus
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That's coy. :)
I am nothing if not coy.