In this question and answers (How was Euler able to create an infinite product for sinc by using its roots?) we use the fact that the real roots of $f(x)=\sin x$ occur when $x$ is an integer multiple of $\pi$ to obtain an infinite product for $\sin x$ in terms of of its factors.
My question is, how do we know that $f(x)=\sin x$ has no non-real roots?
Thanks in advance.
Best Answer
$$\sin(x+iy)=\sin x\cosh y+i\cos x\sinh y$$ Note that $\cosh y$ is never zero for $y$ a real number. Thus, if $\sin(x+iy)=0$, $\sin x=0$ (the real and imaginary parts must be identically zero). But this forces $\cos x=\pm1$, which in turn forces $\sinh y=0$ and thus $y=0$. All the roots must be real.