solve in R the equation

Algebra Level 2

log x log 2 + log ( x + 1 ) log 3 = 2 \frac {\log x}{\log 2} + \frac {\log (x+1)}{\log 3} =2

Solve for the real x x in the equation above.


The answer is 2.

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3 solutions

Patrick Corn
Dec 3, 2019

If x > 2 x > 2 then log ( x ) / log ( 2 ) > 1 \log(x)/\log(2) > 1 and log ( x + 1 ) / log ( 3 ) > 1 , \log(x+1)/\log(3) > 1, so the left side is too big.

If x < 2 x < 2 then log ( x ) / log ( 2 ) < 1 \log(x)/\log(2) < 1 and log ( x + 1 ) / log ( 3 ) < 1 , \log(x+1)/\log(3) < 1, so the left side is too small.

If x = 2 x=2 then the equation is clearly true. Hence the only solution is 2 . \fbox{2}.

Elijah L
Dec 3, 2019

By inspection, x = 2 x=2 .

log x log 2 + log ( x + 1 ) log 3 = 2 Multiply both sides by log 2 log 3 log x log 3 + log ( x + 1 ) log 2 = 2 log 2 log 3 Putting x = 2 log 2 log 3 + log 3 log 2 = 2 log 2 log 3 \begin{aligned} \frac {\log x}{\log 2} + \frac {\log(x+1)}{\log 3} & = 2 & \small \blue{\text{Multiply both sides by }\log 2 \log 3} \\ \log x \log 3 + \log (x+1) \log 2 & = 2\log 2\log 3 & \small \blue{\text{Putting }x=2} \\ \log 2 \log 3 + \log 3 \log 2 & = 2 \log 2 \log 3 \end{aligned}

Therefore, x = 2 x= \boxed 2 .

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