Finding direction after adding/subtracting vectors?

[csgirl504]

Homework Statement

For the vectors given in Fig. 3-32 (|A| = 67.0 and θ = 55.5°), determine the following.

Image for the problem (http://www.webassign.net/giancoli5/3_35alt.gif)

Need to find:

C-A-B

Direction (counterclockwise from the +x axis is positive)

Homework Equations

The Attempt at a Solution

I understand how to find magnitude when adding and subtracting vectors. And I understand that the angle is found by tan^-1 (ry/rx). I got 117 for the magnitude. My rx is -36.5 and my ry is -111.22. So I took the inverse tangent and got 71.8, which is incorrect. I know that sometimes you have to subtract that angle from 180 or something, but I honestly don't know why or when you do that.

If anyone can explain this to me, I would greatly appreciate it!

 

[SammyS]

Welcome to PH Forums.

Make a rough sketch of C + (-A) + (-B) to give you an idea of whether you need to add or subtract 180° .