From Coroneos,
Let \; f(\theta )=cos\theta + isin \theta \\ \therefore f'(\theta)=-sin\theta+icos\theta=i(cos\theta+isin\theta)=if(\theta) \\ i.e \; \frac{f'(\theta)}{f(\theta)}=i, and \therefore \int\frac{f'(\theta)}{f(\theta)}= \int i d \theta \\ ln(f(\theta))= i\theta + C \\ When...