Solve for x by J. Sacramento

Algebra Level 3

x + log 10 ( 1 + 2 x ) = x log 10 ( 5 ) + log 10 ( 6 ) \large x + \log_{10} \big(1 + 2^x \big) = x \log_{10} (5) + \log_{10} (6)

Given the above, solve for x x .


The answer is 1.

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2 solutions

Chew-Seong Cheong
Jan 31, 2018

x + log 10 ( 1 + 2 x ) = x log 10 5 + log 10 6 log 10 1 0 x + log 10 ( 1 + 2 x ) = log 10 5 x + log 10 6 Note: a log b = log b a log 10 ( 1 0 x ( 1 + 2 x ) ) = log 10 ( 6 ( 5 x ) ) Note: log a + log b = log ( a b ) 1 0 x ( 1 + 2 x ) = 6 ( 5 x ) Divide both sides by 5 x 2 x ( 1 + 2 x ) = 6 ( 2 x ) 2 + 2 x 6 = 0 A quadratic equation of 2 x . ( 2 x + 3 ) ( 2 x 2 ) = 0 2 x 2 = 0 2 x + 3 = 0 has no real solution. x = 1 \begin{aligned} x + \log_{10} (1+2^x) & = x \log_{10} 5 + \log_{10} 6 \\ \log_{10} 10^x + \log_{10} (1+2^x) & = \log_{10} 5^x + \log_{10} 6 & \small \color{#3D99F6} \text{Note: } a\log b = \log b^a \\ \log_{10} \left(10^x (1+2^x)\right) & = \log_{10} \left(6(5^x)\right) & \small \color{#3D99F6} \text{Note: } \log a + \log b = \log (ab) \\ 10^x (1+2^x) & = 6(5^x) & \small \color{#3D99F6} \text{Divide both sides by }5^x \\ 2^x (1+2^x) & = 6 \\ (2^x)^2 + 2^x - 6 & = 0 & \small \color{#3D99F6} \text{A quadratic equation of }2^x. \\ (2^x + 3)(2^x-2) & = 0 \\ 2^x - 2 & = 0 & \small \color{#3D99F6} 2^x+3=0 \text{ has no real solution.} \\ \implies x & = \boxed{1} \end{aligned}

Jose Sacramento
Jan 31, 2018

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