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N468989

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i had a class where my teacher was talking about this but i don't know where this comes from, if anyone knows could you please give hint on the type of material this is. Like what type of mathbook i should be looking for.

the problem was:

Demonstrate the following expression:

[tex]\[\displaystyle\sum_{n=0}^{N-1} \alpha^n = \left\{

\begin{array}{l l}

N & \quad \mbox{, $\alpha$ = 1}\\

\frac{1-\alpha^N}{1-\alpha} & \quad \mbox{, for any complex, $\alpha$ $\neq$ 1}\\ \end{array} \right. \][/tex]

thankyou very much for any help.