# Mastering Physics Vector Question

1. Jan 26, 2009

### fishguts

1. The problem statement, all variables and given/known data

I am having a problem with a mastering physics problem. I am given this image

I am to Express C in terms of vector A, vector B, and angle theta, using radian measure for known angles. I know theta is equal to pi - c.

2. Relevant equations

law of cosines

3. The attempt at a solution

By using the law of cosines the answer i got was

C = sqrt(A^2 + B^2 - 2ABcos(θ))

The feedback I got back from the program was to check my signs but everything apperas to be correct. Can someone help me out?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 26, 2009

### muscaria

Hi,
Should'nt it be C = sqrt(A^2 + B^2 - 2ABcos(Pi-θ)) ?
As c is the angle opposite to C.

3. Jan 26, 2009

### fishguts

Thats right! I do not know how I missed that

4. Jan 26, 2009

### fishguts

I am having trouble with the second part of the problem. I do not know if it is my input to the problem or if I have the problem totally wrong. I am told to:

Find the angle phi that the vector C_vec makes with vector A_vec.

I get arcsin(ABsin(pi - theta) / Csin(a))

For earlier solutions in the problem I found C = to pi - theta so U used that instad of c. Can anyone help me out?

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