1. The problem statement, all variables and given/known data This is an easy one, but keep in mind I'm kind of a newbie, anyway I can't figure out how to get the next formula... tan(z) = (tan(a)+i tanh(b))/(1 - i tan(a)tan(b)) 2. Relevant equations This is the third part of an excercise, previous I proof the follow, -all using the definitions, of complex sin,cos, and tan, and definitions of real sinh,cosh, and tanh-... cos(z)=cos(a)cosh(b)-i sin(a)sinh(b); sin(z)=sin(a)cosh(b)+icos(a)sinh(b) 3. The attempt at a solution I tried a lot of things, but couldn't get any way, maybe I'm missing something important, and that's what I fear. I tried to replace tan(z) = sin(z)/cos(z) with the other equations but, I'm getting nothing. Sorry for my english, lot of thanks for the help!