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Math Help - Using Law of Cosines

  1. #1
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    Using Law of Cosines

    Here is the problem (It's actually an example in a review book... Their explanation is insufficient for me.) Find the number of degrees in the largest angle of a triangle whose sides are 3, 5, and 7.

    They then use the law of cosines and come up with cosC=-1/2. Since cosC<0 and <C is an angle of a triangle, the angle must be between 90 degrees and 180 degrees.. Their conclusion is "Therefore, <C =120 degrees."

    I get everything up until this conclusion... Why, therefore, is it 120 degrees?

    Side note: When graphing, how do you get the TI83 to immediately show the graph and not "draw it out" and waste 10 seconds?
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  2. #2
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    Re: Using Law of Cosines

    Because \displaystyle \cos^{-1}{\left(-\frac{1}{2}\right)} = 120^{\circ}...
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    Re: Using Law of Cosines

    Quote Originally Posted by Prove It View Post
    Because \displaystyle \cos^{-1}{\left(-\frac{1}{2}\right)} = 120^{\circ}...
    Thank you...

    How do you type your math symbols?
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    Re: Using Law of Cosines

    Quote Originally Posted by benny92000 View Post
    Thank you...

    How do you type your math symbols?
    LaTeX, there's a subforum here about it. You basically learn the code for the maths symbols you want to type, and type them inside "Tex" tags.
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