Logarithm Basic (2)

Algebra Level 1

9 2 x 4 = 27 x , x = ? \large {9}^{2x-4}={27}^{x} \quad, \quad x = \ ?


The answer is 8.

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8 solutions

Abhishek Singh
Jun 8, 2014

9 2 x 4 = 2 7 x 9^{2x-4}=27^{x} can be written as 3 2 ( 2 x 4 ) = 3 3 x 3^{2(2x-4)}=3^{3x} now taking log both sides with base 3 log 3 3 2 ( 2 x 4 ) = log 3 3 3 x \log_{3} {3^{2(2x-4)}}=\log_{3} {3^{3x}} which simply gives 4 x 8 = 3 x 4x-8=3x hence x = 8 x=\boxed{8}

dude you don't need to use logarithm,

Alex Thang - 6 years, 11 months ago

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Taking logarithm on both sides to bring the powers down is a more logical way than just comparing L.H.S and R.H.S. That's the reason why he used logarithm!

Prasun Biswas - 6 years, 11 months ago

9 2 x × 9 4 = 2 7 x 3 4 x × 3 8 = 3 3 x 3 4 x 8 = 3 3 x 9^{2x}\times 9^{-4}=27^{x}\\3^{4x}\times 3^{-8}=3^{3x}\\3^{4x-8}=3^{3x} Equating 4 x 8 = 3 x x = 8 4x-8=3x\implies x=\boxed{8}

Mohammad Saleem
Jul 12, 2015

Kenny Lau
Jul 7, 2014

(This solution uses logarithm because the topic says so) 9 2 x 4 = 2 7 x log 3 9 2 x 4 = log 3 2 7 x ( 2 x 4 ) log 3 9 = x log 3 27 2 ( 2 x 4 ) = 3 x 4 x 8 = 3 x x 8 = 0 x = 8 \begin{array}{rcl} 9^{2x-4}&=&27^x\\ \log_39^{2x-4}&=&\log_327^x\\ (2x-4)\log_39&=& x\log_327\\ 2(2x-4)&=&3x\\ 4x-8&=&3x\\ x-8&=&0\\ x&=&8 \end{array}

Kyle Chui
Dec 15, 2014

It took me a while to figure out what the question was lol ;)

3^(4x - 8) = 3^3x 4x - 8 = 3x x = 8

William Isoroku
Jul 29, 2014

Just use the rules of logs.

Kevin Patel
Jul 16, 2014

Equating, you get 4x - 8 = 3x
thus, x = 8

Nha Pham
Jul 13, 2014

\­( { 27 }^{ x }\quad =\quad { 9 }^{ x }\quad \times \quad { 3 }^{ x }\quad =\quad { 9 }^{ x }\quad \times \quad { 9 }^{ 1/2\quad x }\quad =\quad { 9 }^{ 3/2\quad x }\ \gg \quad 2x\quad -\quad 4\quad =\quad 3/2\quad x\quad \ \gg \quad x\quad =\quad 8 \­)

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