Hi, I know this is baby/fetus/sperm/molecule math for all of you, but I'm 13 and trying to self study my way to physics. Anyway, I'm teaching myself pre calculus. Most of it is pretty easy, but I've been stuck on sum/difference identities. It seems clear, and I'm folliwing everything the textbook is telling me. However, I think I'm making stupid mistakes. For example: Question: tan(4pi/3+5pi/4) I worked out the format of the tangent sum of two angles, and: (Tan(4pi/3)+tan(5pi/4) )/(1-tan(4pi/3)tan(5pi/4)) =(sqrt(3)+1)/(1-(sqrt(3))) That was my answer Book answer:-sqrt(3)-2 Please tell me what stupid mistake I'm making if you can. I'm probably really embarrassing myself.