# Formula for angle between 2 lines

• Nov 25th 2011, 11:33 AM
mathminor827
Formula for angle between 2 lines
Hi ---

I can't figure out the yellow and red
parts of the solution to the problem below.

That picture at the bottom is mine. I drew it to try to help me but I am still
confused.

http://img833.imageshack.us/img833/6334/11201.jpg

Yellow: Where did they get $180 - (\theta_2 - \theta_1)$ from? Why
Red: If $\theta_2 < \theta_1$ why is $\alpha = 180 - (\theta_2 - \theta_1)$?
Yellow - You have it marked. Your $\theta_{1} - \theta_{2}$ is nicely labeled. What would you label the next angle - continuing anti-clockwise? It begins on your blue ray and ends with the portion of the red ray extended through the origin.
Red - This is just a second case. If you changed $\theta_{1}$ to 160º, would $\theta_{1} - \theta_{2}$ give an acute angle?