# Using the law of cosines [closed]

I am supposed to find the value of $\theta$ in a triangle. I am given a triangle with 3 lengths and no angles. The bottom left is $\theta$, bottom line is 5, right is 7 and left is 3. I have no idea what to do. My book used some strange forumla $\cos(\theta)= (b^2 + c^2 - a^2)/2bc$.

How do I know what is a $b$ and $c$? i tried it and got the wrong answer, only one combination of abc works. I need to memorize this forumla for a test tomorrow, along with the 3 other cosine laws, and the 3 sin laws and the 21 other half and double identities, and the cofunctions.

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## closed as too localized by t.b., yunone, Andres Caicedo, Arturo Magidin, Qiaochu YuanJul 3 '11 at 2:28

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I'm voting to close this question. Adam: you asked 35 questions and still are unable to typeset your questions. You still haven't registered and haven't upvoted a single answer you were given. You're using strong language offending many people here, repeatedly, and after being told many times to cut it out. I find this attitude unbearable. –  t.b. Jul 1 '11 at 0:46
Thanks friends, I guess I will just fail this class and switch degrees. –  Adam Jul 1 '11 at 0:48
Dear Adam, please change your attitude. If you are seriously thinking about failing this class and switching degrees simply if you do not receive help here, then this is worrying. –  Amitesh Datta Jul 1 '11 at 1:25