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Algebra Level 4

log a ( x 2 x + 2 ) > log a ( x 2 + 2 x + 3 ) \log_a(x^2 - x + 2) > \log_a(-x^2 + 2x + 3)

If x = 4 9 x = \frac{4}{9} satisfies the above inequality, then find the sum of all integer solutions of x x to the inequality.


Try my set .


The answer is 1.

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1 solution

A very important property that one should know before proceeding with the problem

If log c d > log c f \log_cd > \log_cf and d < f d < f , then it can be easily inferred that the base c c is less than 1 1 .

Substituting x = 4 9 x = \dfrac{4}{9} in the inequation, we get

log a 142 81 > log a 299 81 \log_a\dfrac{142}{81} > \log_a\dfrac{299}{81}

a ( 0 , 1 ) \Rightarrow a \in (0,1)

So when we remove the logarithm the inequality is reversed.

x 2 x + 2 < x 2 + 2 x + 3 2 x 2 3 x 1 < 0 x^2 - x + 2 < -x^2 + 2x + 3 \Rightarrow 2x^2 - 3x - 1 < 0

The solution set to the above inequality is x ( 3 17 4 , 3 + 17 4 ) ( 1 ) x \in \left( \dfrac{3-\sqrt{17}}{4} , \dfrac{3+\sqrt{17}}{4} \right) \ldots (1)

But we simply cannot conclude this is possible without checking the domains of the logarithms in the R.H.S and L.H.S

For the R.H.S logarithm to exist,

x 2 + 2 x + 3 > 0 x 2 2 x 3 < 0 x ( 1 , 3 ) ( 2 ) -x^2 + 2x + 3 > 0 \Rightarrow x^2 - 2x - 3 < 0 \Rightarrow x \in (-1,3) \ldots (2)

For the L.H.S logarithm to exist,

x 2 x + 2 > 0 ( x 1 2 ) 2 + 7 4 > 0 x R ( 3 ) x^2 - x + 2 > 0 \Rightarrow \left(x-\dfrac{1}{2}\right)^2 + \dfrac{7}{4} > 0 \Rightarrow x \in \mathbb{R} \ldots (3)

Considering the intersection of ( 1 ) , ( 2 ) & ( 3 ) (1),(2) \text{ \& } (3) , we get

x ( 3 17 4 , 3 + 17 4 ) x ( 0.280 , 1.780 ) x \in \left( \dfrac{3-\sqrt{17}}{4} , \dfrac{3+\sqrt{17}}{4} \right) \Rightarrow x \in (-0.280 , 1.780)

The integral values of x x in the given interval are 0 , 1 \boxed{0},\boxed{1}

Sum of the values is 1 \large \color{#D61F06}{\boxed{1}}

Moderator note:

Great question! How did you come up with it?

The "very important property" is often forgotten. It is very common to assume that the logarithm curve is always increasing. That is not the case.

I did not come up with this question. My maths teacher gave this question during JEE Prep.

Vishwak Srinivasan - 5 years, 11 months ago

I did not want to substitute the 4/9 so I just went ahead and solved to get the roots. As 4/9 clearly lies between the roots, the values of x which must be chosen are also between the roots!

Ajinkya Shivashankar - 4 years, 7 months ago

Overrated!!!

Naman Kapoor - 5 years, 11 months ago

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