+118677 I will assume that you mean
f(x) = sin3xcosx/(tan2x+1) this has no global maximum (ref: Wolframalpha)
so maybe you mean
f(x) = (sin3xcosx/tan2x)+1 (yes I know this is how it should be interpreted bum a loy of people make mistakes here)
I spent ages working on the first interpretation, I haven't got the energy to look at this second one at present but Wolfram Alpha says the maximum is 2.5
