  Let $S$ be a semigroup. Our main results is that we describe the
complex-valued solutions of the following functional equations \[g(x\sigma (y))
= g(x)g(y)+f(x)f(y),\ x,y\in S,\] \[f(x\sigma (y)) = f(x)g(y)+f(y)g(x),\ x,y\in
S,\] and \[f(x\sigma (y)) = f(x)g(y)-f(y)g(x),\ x,y\in S,\] where $\sigma
:S\rightarrow S$ is an automorphism that need not be involutive. As a
consequence we show that the first two equations are equivalent to their
variants. We also give some applications.
