Hi, I'm studying for my pre-calc test tomorrow and I've run into a snag. I just can't seem to get the correct answer for a Tangent sum problem, and I'm hoping someone could help me out with it. It goes like this: Sum Formula for Tangant: Tan (a+b) = Tan a + Tan b/1-Tan a * Tan b Problem: Find the exact value, Tan 15 + Tan 30/1- Tan 15 * Tan 30 Correct answer: -1 but I can't seem to get the answer in the end; are the tangants in the numerator added to make tan 45 then solved for? If not, how would you express tan 15 in an exact form? Similarly, how do you multiply tan 15 and tan 30? Or am I missing some other identity? Its been driving me crazy all day!