# Thread: Using Law of Cosines

1. ## 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?

2. ## Re: Using Law of Cosines

Because $\displaystyle \cos^{-1}{\left(-\frac{1}{2}\right)} = 120^{\circ}$...

3. ## Re: Using Law of Cosines

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

How do you type your math symbols?

4. ## Re: Using Law of Cosines

Originally Posted by benny92000
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.