What's with these logs?

Algebra Level 2

ln ( 2 x 2 3 x + 8 ) ln ( x 4 + 4 x 3 + 8 x 2 + 8 x + 4 ) 2 \ln\left(2x^{2} - 3x + 8\right) \le \dfrac{\ln\left(x^{4}+4x^{3} + 8x^{2} + 8x + 4\right)}{2}

Find the product of all integer values of x x which satisfy the inequality above.


The answer is 6.

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1 solution

ln ( 2 x 2 3 x + 8 ) ln ( x 4 + 4 x 3 + 8 x 2 + 8 x + 4 ) 2 \ln\left(2x^{2} - 3x + 8\right) \le \dfrac{\ln\left(x^{4}+4x^{3} + 8x^{2} + 8x + 4\right)}{2}
ln ( 2 x 2 3 x + 8 ) ln ( x 2 + 2 x + 2 ) 2 2 \ln\left(2x^{2} - 3x + 8\right) \le \dfrac{\ln\left(x^{2}+2x+2\right)^{2}}{2}
The determinants of both the quadratics are negative, and the leading co-efficient is greater than 0, thus they are always greater than 0, and the logarithmic function is well defined.
ln ( 2 x 2 3 x + 8 ) ln ( x 2 + 2 x + 2 ) \ln\left(2x^{2} - 3x + 8\right) \le\ln\left(x^{2}+2x+2\right)
2 x 2 3 x + 8 x 2 + 2 x + 2 \therefore 2x^{2} - 3x + 8 \le x^{2} + 2x + 2
x 2 5 x + 6 0 x^{2} - 5x + 6 \le 0
( x 2 ) ( x 3 ) 0 (x-2)(x-3) \le 0

Drawing the wavy curve we get our interval as,
x [ 2 , 3 ] x \in [2,3]
The only integer values that satisfy this are 2 , 3 2,3
2 3 = 6 2\cdot 3 = 6
Pardon my weirdly drawn wavy curve.


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