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.