Let 123 111 , , ,….., xxx (x i ¹ 0 for i = 1, 2, …., n) be in A.P. such that x 1 = 4 and x 21 = 20. If n is the least positive integer for which x n > 50, then 1 1 = æ ö ç ÷ è ø å n i i x is equal to.

Let $\frac{1}{x_{1}}, \frac{1}{x_{2}}, \frac{1}{x_{3}}, \ldots . .,\left(x_{i} \neq 0\right.$ for $\left.i=1,2, \ldots, n\right)$ be in A.P. such that $x_{1}=4$ and $x_{21}=20$. If $n$ is the least positive integer for which $x_{n}>50$, then $\sum_{i=1}^{n}\left(\frac{1}{x_{i}}\right)$ is equal to.