(1+x)^n=\sum_{k=0}^n \binom{n}{k}x^k
=\binom{n}{0}x^0+\binom{n}{1}x^1+\binom{n}{2}x^2+\binom{n}{3}x^3+\dots+\binom{n}{n-1}x^{n-1}+\binom{n}{n}x^n
=1+\frac{n}{1!}x+\frac{n(n-1)}{2!}x^2+\frac{n(n-1)(n-2)}{3!}x^3+\dots+\binom{n}{n-1}x^{n-1}+\binom{n}{n}x^n
x=\frac{1}{n} koyalim
(1+\frac{1}{n})^n=1+\frac{1}{1!}+\frac{(n-1)}{2!n}+\frac{(n-1)(n-2)}{3!n^2}+\dots+\binom{n}{n-1}(\frac1n)^{n-1}+\binom{n}{n}(\frac1n)^n
=1+\frac{1}{1!}+\frac{1}{2!}+\frac{-1}{2!n}+\frac{1}{3!}+\frac{-3}{3!n}+\frac{2}{3!n^2}+\dots+\binom{n}{n-1}(\frac1n)^{n-1}+\binom{n}{n}(\frac1n)^n
e=\lim_{n \rightarrow\infty}(1+\frac{1}{n})^n\\ =\lim_{n \rightarrow\infty}\left(1+\frac{1}{1!}+\frac{1}{2!}+\frac{-1}{2!n}+\frac{1}{3!}+\frac{-3}{3!n}+\frac{2}{3!n^2}+\dots+\binom{n}{n-1}(\frac1n)^{n-1}+\binom{n}{n}(\frac1n)^n \right)
=1+\frac{1}{1!}+\frac{1}{2!}+\frac{1}{3!}+\dots=\sum_{k=0}^{\infty}\frac{1}{k!}