Because ...
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?