Inequality -- 4

Level pending

Knowing function f ( x ) = 4 x 2 + 2 x + a + 2 f(x) = 4x^2+2x+a+2 . When x [ 1 , ) x \in [1, \infty) , f ( x ) 2 a x f(x) \geq 2ax , what would be the maximum value for a a ?


The answer is 7.

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1 solution

Kevin Xu
Sep 6, 2019

4 x 2 + 2 x + a + 2 2 a x 4x^2+2x+a+2 \geq 2ax \\ 4 x 2 + 2 x + 2 ( 2 x 1 ) a 4x^2+2x+2 \geq (2x-1)a \\ x [ 1 , ) \because x\in [1, \infty) 2 x 1 1 \therefore 2x-1 \geq 1 \\ \quad \\ a 4 x 2 + 2 x + 2 2 x 1 \therefore a \leq \frac {4x^2+2x+2}{2x-1} \quad [The key is to create a equation of a using only variable x] \\
a ( 2 x 1 ) 2 + 3 ( 2 x 1 ) + 4 2 x 1 a \leq \frac {(2x-1)^2 + 3(2x-1)+4}{2x-1} \\ a ( 2 x 1 ) + 4 2 x 1 + 3 a \leq (2x-1)+\frac {4}{2x-1} + 3 \\ a 2 4 + 3 a \leq 2 \sqrt 4 + 3 \\ a 7 a \leq 7

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