Why not changing bases?

Algebra Level 5

If the real values of x which satisfy: 4 log 3 ( x 2 ) 2 + log x ( 1 / 9 ) > 1 \frac{4}{\log_3 (x^2) -2} + \log_x (1/9) > 1 are on the interval X = ] a , b [ U ] c , d [ X = \left]a,b\right[\ U \left]c, d\right[ , find the value of 3 a + b + c + d . 3a+b+c+d.

Obs: a<b<c<d


The answer is 14.

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1 solution

Sanjeet Raria
Nov 17, 2014

4 log 3 ( x ) 2 2 + log x 1 9 > 1 \frac{4}{\log_3(x)^2-2}+ \log_x \frac{1}{9}>1 2 log 3 x 1 2 log 3 x > 1 \Rightarrow \frac{2}{\log_3 x-1}-\frac{2}{\log_3x}>1

Now putting log 3 x = y \log_3 x=y we get 2 y 1 2 y 1 > 0 \frac{2}{y-1}-\frac{2}{y}-1>0 ( y 2 ) ( y + 1 ) y ( y 1 ) < 0 \Rightarrow \frac{(y-2)(y+1)}{y(y-1)}<0 y ( 1 , 0 ) ( 1 , 2 ) \Rightarrow y \in (-1,0)\cup (1,2) x ( 1 3 , 1 ) ( 3 , 9 ) \Rightarrow x \in (\frac{1}{3},1) \cup (3,9) Answer is 1 + 1 + 3 + 9 = 14 1+1+3+9=\boxed{14}

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