Sum of Binomial Coefficients over Lower Index/Proof 3

Proof
From the Binomial Theorem, we have that:


 * $\displaystyle \forall n \in \Z_{\ge 0}: \left({x + y}\right)^n = \sum_{i \mathop = 0}^n \binom n i x^{n - i}y^i$

Putting $x = y = 1$ we get: