Power Mean Inequalities

Algebra Level 3

a 1 2 + 4 a 1 + a 2 2 + 4 a 2 + + a n 2 + 4 a n 2017 \large\frac {a_{1}^2 + 4}{a_1} + \frac {a_{2}^2 + 4}{a_2} + \cdots + \frac {a_{n}^2 + 4}{a_n} \geq 2017

Find the least positive integer value of n n such that for any n n positive reals a 1 , a 2 , , a n a_{1}, a_{2}, \ldots , a_{n} , the inequality above is satisfied.


The answer is 505.

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1 solution

Kushal Bose
Dec 28, 2016

Simplifying the expression ( a 1 + a 2 + a 3 + . . . . . . . . + a n ) + 4 ( 1 a ! + 1 a 2 + 1 a 3 + . . . . . . . + 1 a n ) 2017 = > i = 1 n ( a i + 4 a i ) 2017 (a_1+a_2+a_3+........+a_n) + 4 ( \dfrac{1}{a_!}+ \dfrac{1}{a_2} + \dfrac{1}{a_3}+.......+\dfrac{1}{a_n}) \geq 2017 \\ \\ => \sum_{i=1}^{n} (a_i +\dfrac{4}{a_i}) \geq 2017

Now applying A.M.-G.M. a i + 4 a i 2 a i × 4 a i = 4 a_i+ \dfrac{4}{a_i} \geq 2 \sqrt{a_i \times \dfrac{4}{a_i}}=4

We will find the minimum value of n n .The n n will minimum when it will holds the equality.So each a i + 4 a i a_i+ \dfrac{4}{a_i} will be 4 4

So, putting this value i = 1 n 4 2017 4 n 2017 n 2017 / 4 = 504.25 \sum_{i=1}^{n} 4 \geq 2017 \implies 4n \geq 2017 \implies n \geq 2017/4=504.25

The next integer n = 505 n=505 will be the minimum value.

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