1989 APMO Problems/Problem 1

Problem

Let $x_1$, $x_2$, $\cdots$, $x_n$ be positive real numbers, and let \[S = x_1 + x_2 + \cdots + x_n.\] Prove that \[(1 + x_1)(1 + x_2) \cdots (1 + x_n) \leq 1 + S + \frac{S^2}{2!} + \frac{S^3}{3!} + \cdots + \frac{S^n}{n!}\]

Solution