A recursive problem

Calculus Level 2

x n + 1 = 5 6 x n , n 1 \large x_{n+1}=\frac{5}{6-x_n}, \quad n \ge 1

Given that x 1 = 2 x_1=2 , find the value of lim n x n \displaystyle \lim_{n\to \infty} x_n .

4 3 5 2 1 1 or 5

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1 solution

Chan Lye Lee
Mar 29, 2017

Let lim n x n = a \lim_{n \to \infty} x_n =a . From x n + 1 = 5 6 x n x_{n+1}=\frac{5}{6-x_n} , we have a = 5 6 a a=\frac{5}{6-a} , which means that a 2 6 a + 5 = 0 a^2-6a+5=0 . Solving the equation we obtain a = 1 a=1 or a = 5 a=5 .

As x 1 = 2 < 3 x_1=2<3 , we have x 2 = 5 6 x 1 < 5 3 < 3 x_2=\frac{5}{6-x_1}<\frac{5}{3}<3 . By similar argument, x n < 3 x_n<3 for all n 1 n\ge 1 . Hence a 5 a \neq 5 and therefore a = 1 a=1 .

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