e^x = \sum\limits_{i = 0}^\infty {\frac{{x^i }}{{i!}}} = 1 + \frac{x}{{1!}} + \frac{{x^2 }}{{2!}} + \frac{{x^3 }}{{3!}} + \frac{{x^4 }}{{4!}} + \frac{{x^5 }}{{5!}} + ...