f(x, y, z) ??? Hey there, Sorry for the title, but i cannot find any appropriate one for it, here is my question! If f(x, y, z) = tanh (x+2y+3z) => fx =?(what is the derivative of f(x, y, z) with respect to x?) Actually, i know the answer but i do not get the idea! In case it might help, the answer is that: sech^2(x+2y+3z) So, as i said, can someone explain it to me? How can they reach this answer? Thanks!