Fix missing i in expression for sine. Closes #5

src/1_complex_numbers.md

@@ -173,7 +173,7 @@ As a result, $y$ is only defined up to $2\pi$.
 
 Furthermore, we can define the sine and cosine in terms of complex exponentials:
 $$\cos(x) = \frac{e^{{\rm i} x} + e^{-{\rm i} x}}{2}$$
-$$\sin(x) = \frac{e^{{\rm i} x} - e^{-{\rm i} x}}{2}$$
+$$\sin(x) = \frac{e^{{\rm i} x} - e^{-{\rm i} x}}{2i}$$
 
 Most operations on complex numbers are easiest when converting the complex number to its *polar form*, using the exponential.
 Some operations which are common in real analysis are then easily derived for their complex counterparts: