Properties of Logarithms

Algebra Level pending

x x is a real number that satisfies the following equation: log 2 ( x 5 ) = log 4 ( x 2 ) + 1 \log_2{(x-5)} = \log_4{(x-2)} + 1 . What is the value of x x ?


The answer is 11.

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1 solution

Arron Kau Staff
May 13, 2014

We can rewrite log 4 ( x 2 ) \log_4{(x-2)} as

log 4 ( x 2 ) = log 2 ( x 2 ) log 2 4 = 1 2 log 2 ( x 2 ) . \log_4{(x-2)} = \frac{\log_2{(x-2)}}{\log_2{4}} = \frac{1}{2} \log_2{(x-2)}.

Since 1 = log 2 2 1 = \log_2{2} , the given equation can be rewritten as

log 2 ( x 5 ) = 1 2 log 2 ( x 2 ) + log 2 2. \log_2{(x-5)} = \frac{1}{2} \log_2{(x-2)} + \log_2{2}.

Multiplying 2 on both sides, rearranging terms, and factorizing, we have

log 2 ( x 5 ) = 1 2 log 2 ( x 2 ) + log 2 2 2 log 2 ( x 5 ) = log 2 ( x 2 ) + log 2 4 log 2 ( x 5 ) 2 = log 2 4 ( x 2 ) ( x 5 ) 2 = 4 ( x 2 ) 0 = x 2 14 x + 33 0 = ( x 3 ) ( x 11 ) . \begin{aligned} \log_2{(x-5)} &= \frac{1}{2} \log_2{(x-2)} + \log_2{2} \\ 2\log_2{(x-5)} &= \log_2{(x-2)} + \log_2{4} \\ \log_2{(x-5)^2} &= \log_2{4(x-2)} \\ (x-5)^2 &= 4(x-2) \\ 0 &= x^2-14x+33 \\ 0 &= (x-3)(x-11). \\ \end{aligned}

Thus, x = 3 x=3 or x = 11 x=11 . However, x = 3 x=3 cannot be a solution because x 5 = 2 < 0 x-5 = -2 < 0 , and the domain of the logarithm function is positive numbers.

Therefore, x = 11. x = 11.

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