## anonymous one year ago Find the direction angle of v for the following vector. v = -7i - 9j Ok, since we can write this as (-7,-9) and this is a triangle we can use tan a = -9/-7 tan a = 9/7 Ok I need to solve. How do I do that. What I keep coming up with is wrong. Do I just user the $$tan^{-1}(9/7)$$

1. anonymous

I need it in degrees

2. anonymous

$$tan^{-1}(9/7)$$ is suppose to be $$\huge tan^{-1} (\frac{9}{7})$$

3. anonymous

and once I find that subtract 360?

4. anonymous

@peachpi

5. anonymous

@math1234

6. anonymous

Are you getting 52.1°?

7. anonymous

Yes, 52.125

8. anonymous

Then I subtract that by 360, which gives 307.87

9. anonymous

But this is wrong.

10. anonymous

I think it has something to do with the quadrant it is in.

11. anonymous

52.125 is the angle in the 1st quadrant. (-7, -9) is in the 3rd quadrant. You need to add 180.

12. anonymous

Ah that is it. I knew it had to do with the quadrant lol

13. anonymous

Yep that was it. I was working with it as in the 4th quadrant.

14. anonymous

ah ok. it's hard to keep those straight