# discrete Fourier transform

Hi All, while studying the discrete Fourier transform, I came across this equality which I can't quite explain : $\sum_{n=0}^{N-1} e^{-i2\pi\phi n } = \frac{1-e^{-i2\pi \phi N} } {1 - e^{-i2\pi \phi}}$ How does a finite sum of complex exponentials lead to the fraction on the right hand side ? Thanks for any help Cheers