Typo - Sine in terms of complex exponential

File in which the problem appears

src/1_complex_numbers.md

Also in the summary of the chapter.

Problematic sentence

Furthermore, we can define the sine and cosine in terms of complex exponentials:

\sin(x) = \frac{e^{{\rm i} x} - e^{-{\rm i} x}}{2}

Correct version

\sin(x) = \frac{e^{{\rm i} x} - e^{-{\rm i} x}}{2{\rm i}}