I'm going through a book about algorithms and I encounter this.
$\frac {b^{n+1}-a^{n+1}}{b-a} = \sum_{i=0}^{n}a^ib^{n-i}$
How is this equation formed? If a theorem has been applied, what theorem is it?
[Pardon me for asking such a simple question. I'm not very good at maths.]