Variable Base Logarithm

Algebra Level 2

log 3 x + 5 ( 9 x 2 + 8 x + 8 ) > 2 \log _{ 3x+5 }{ \big(9{ x }^{ 2 }+8x+8\big) } >2

Solve the inequality above for x x .


Try Fun with inequalities-1 and Fun with inequalities-3 .
x ( 4 3 , 17 22 ) x\in \left(\frac { 4 }{ 3 } ,\frac { 17 }{ 22 } \right ) x ( 4 3 , 17 22 ) x\in \left(\frac { 4 }{ 3 } ,-\frac { 17 }{ 22 } \right ) x ( 4 3 , 17 22 ) x\in \left(-\frac { 4 }{ 3 } ,\frac { 17 }{ 22 } \right ) x ( 4 3 , 17 22 ) x\in \left(-\frac { 4 }{ 3 } ,-\frac { 17 }{ 22 } \right )

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2 solutions

log 3 x + 5 ( 9 x 2 + 8 x + 8 ) = log ( 9 x 2 + 8 x + 8 ) log ( 3 x + 5 ) > 2 \log_{3x+5} {(9x^2+8x+8)} = \dfrac {\log {(9x^2+8x+8)}}{\log {(3x+5)} } > 2

log ( 9 x 2 + 8 x + 8 ) > 2 log ( 3 x + 5 ) 9 x 2 + 8 x + 8 > ( 3 x + 5 ) 2 9 x 2 + 8 x + 8 > 9 x 2 + 30 x + 25 0 > 22 x + 17 x < 17 22 \begin{aligned} \Rightarrow \log {(9x^2+8x+8)} & > 2 \log {(3x+5)} \\ 9x^2+8x+8 & > (3x+5)^2 \\ 9x^2+8x+8 & > 9x^2+30x+25 \\ 0 & > 22x+17 \\ x & < -\frac{17}{22} \end{aligned}

We note that when log ( 3 x + 5 ) < 0 \log {(3x+5)} < 0 then log ( 9 x 2 + 8 x + 8 ) log ( 3 x + 5 ) < 0 2 \dfrac {\log {(9x^2+8x+8)}}{\log {(3x+5)} } < 0 \not{>} 2 . Therefore, for log ( 9 x 2 + 8 x + 8 ) log ( 3 x + 5 ) > 2 \dfrac {\log {(9x^2+8x+8)}}{\log {(3x+5)} } > 2

log ( 3 x + 5 ) > 0 3 x + 5 > 1 x > 4 3 \Rightarrow \log{(3x+5)} > 0\quad \Rightarrow 3x+5 > 1 \quad \Rightarrow x > -\dfrac {4}{3}

Therefore, the answer: x ( 4 3 , 17 22 ) \boxed {x \in \left(-\frac{4}{3},-\frac{17}{22}\right)}

The graph below clearly shows the answer.

Tapas Pal
Apr 5, 2015

Why is 9x2+8x+8 >1? Whereas we know than the base and the no. Both shud always be greater than 0

riya mukherjee - 2 years, 1 month ago

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