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MathNet 2017-part-07 — 172 problems (EN–VI) Question 86

Math Olympiad Vault · MathNet 2017-part-07 — 172 problems (EN–VI)

Question 86

Let $a_{1}, a_{2}, \ldots$ be an infinite sequence of positive real numbers which satisfies
$$
a_{n+1} \geq a_{n}^{2}+\frac{1}{5}
$$
for every positive integer $n$. Prove that $\sqrt{a_{n+5}} \geq a_{n-5}^{2}$ for each positive integer $n \geq 6$.