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Math Help - Formula for angle between 2 lines

  1. #1
    Junior Member
    Joined
    Sep 2011
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    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.



    Yellow: Where did they get  180 - (\theta_2 - \theta_1) from? Why
    do we care about this?

    Red: If \theta_2 < \theta_1 why is \alpha = 180 - (\theta_2 - \theta_1)?

    Thanks a lot ---
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  2. #2
    MHF Contributor
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    Re: Formula for angle between 2 lines

    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 160, would \theta_{1} - \theta_{2} give an acute angle?
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