# Direction Angle of Vector

1. May 31, 2009

### (-_-)

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

Find the direction angle of vector <-2,-5>

2. Relevant equations

The components of a vector is v=<magnitude*cos ø, magnitude*sin ø>
Magnitude of a vector: √(a^2+b^2)

3. The attempt at a solution

I found the magnitude of the vector, which is √29. Then I set -2= √29 cos ø and solved for ø. It was 111.891 degrees but the answer to the problem was 360 MINUS 111.891 degrees. I thought it would be 180 PLUS 111.891.

2. May 31, 2009

### abhikesbhat

Ok you could draw the point in the plane. Make a right triangle with legs 2,5. You could then arcTan of that. That angle is the angle it makes with negative x-axis and negative y-axis. Since you are measuring from the positive x you add 180 to that. That is the angle.

3. May 31, 2009

### jacksonpeeble

360-111.891 is logical because of where on the coordinate plane you measure from.

By the way, welcome to the forums!

4. May 31, 2009