# Homework Help: Vector Calculation Problem

1. Sep 8, 2008

### Andy111

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

Consider two vectors ~A and ~B and their resul-
tant ~A + ~B . The magnitudes of the vectors ~A
and ~B are, respectively, 12.1 and 7 and they
act at 130◦ to each other.

Use the law of cosines to calculate the mag-
nitude of the resultant vector ~A + ~B.

2. Relevant equations

3. The attempt at a solution

2. Sep 8, 2008

### LowlyPion

How do you figure to apply the Law of Cosines?

3. Sep 8, 2008

### Andy111

I did (A+B)$$^{2}$$=A$$^{2}$$+B$$^{2}$$-2ABcos(130) and solved for (A+B). I got an answer of 17.444148, but that was wrong.

4. Sep 8, 2008

### Redbelly98

Staff Emeritus
Draw a triangle using those vectors.

edit:
This will show you the angle you are supposed to use for the calculation.

5. Sep 8, 2008

### Andy111

I'm a bit lost, why isn't 130 the correct angle to use?

6. Sep 8, 2008

### Redbelly98

Staff Emeritus
Did you draw the triangle?

A picture is worth 1000 words.

7. Sep 8, 2008

### Andy111

Well yes, and I put the angle 130 between A and B, and if that's correct then I don't know how I got this wrong.

8. Sep 8, 2008

### Redbelly98

Staff Emeritus
The correct picture is the 2nd one here.

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9. Sep 9, 2008

### Andy111

Thanks, I see where my mistake was now, I didn't draw the vectors "tip-to-tail".