1953 AMC 12 Problem 47

Attempt Problem 47 of the 1953 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1953 AMC 12 solutions, or check the answer key.

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47.

If xx is greater than zero, then the correct relationship is:

log(1+x)=x1+x\log(1+x)=\dfrac{x}{1+x}

log(1+x)<x1+x\log(1+x)\lt\dfrac{x}{1+x}

log(1+x)>x\log(1+x)\gt x

log(1+x)<x\log(1+x)\lt x

none of these

Answer: D
Concepts:logarithmic inequalityexponential function
Difficulty rating: 1450
Small Hint:

Compare 1+x1+x with an exponential function for x>0x\gt0

Big Hint:

The standard inequality ln(1+x)<x\ln(1+x)\lt x is even stronger for common logarithms

Solution:

For x>0,x\gt0, the standard exponential inequality gives 1+x<ex.1+x\lt e^x. Taking natural logarithms yields ln(1+x)<x. \ln(1+x)\lt x. If log\log denotes the common logarithm, then log(1+x)=ln(1+x)ln10<ln(1+x), \begin{aligned} \log(1+x)&=\frac{\ln(1+x)}{\ln10}\\ &\lt\ln(1+x), \end{aligned} so the same listed inequality holds.

Thus, the correct answer is D.

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