Algebra II : Log-Base-10

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #41 : Logarithms

Solve:  \(\displaystyle 2log(300)\)

Possible Answers:

\(\displaystyle 4log(2)+2\)

\(\displaystyle 2log(3)+2\)

\(\displaystyle 3log(2)+2\)

\(\displaystyle 2log(3)+4\)

\(\displaystyle 3log(2)+4\)

Correct answer:

\(\displaystyle 2log(3)+4\)

Explanation:

Break up \(\displaystyle log(300)\) using log rules.  The log has a default base of ten.

\(\displaystyle log(A\times B) = log(A)+log(B)\)

\(\displaystyle log(300) = log(3)+log(100)=log_{10}(3)+log_{10}(10^2)\)

The exponent can be brought down as the coefficient since the bases of the second term are common.

\(\displaystyle log_{10}(3)+log_{10}(10^2) =log_{10}(3)+2log_{10}(10) = log(3)+2\)

This means that:  \(\displaystyle 2log(300)=2[ log(3)+2] = 2log(3)+4\)

The answer is:  \(\displaystyle 2log(3)+4\)

Example Question #22 : Log Base 10

Evaluate:  \(\displaystyle 2log_{10}(\frac{1}{1000})\)

Possible Answers:

\(\displaystyle \frac{2}{3}\)

\(\displaystyle -3\)

\(\displaystyle \frac{1}{5}\)

\(\displaystyle \frac{1}{50}\)

\(\displaystyle -6\)

Correct answer:

\(\displaystyle -6\)

Explanation:

We will need to write fraction in terms of the given base of log, which is ten.

\(\displaystyle 2log_{10}(\frac{1}{1000})=2log_{10}(10^{-3})\)

According to the log rules:

\(\displaystyle log_{x}(x^Y) =Y\)

This means that the expression of log based 10 and the power can be simplified.

\(\displaystyle 2log_{10}(10^{-3}) =2\cdot -3 = -6\)

The answer is:  \(\displaystyle -6\)

Example Question #281 : Mathematical Relationships And Basic Graphs

Evaluate \(\displaystyle \log_{8}172\) to the nearest tenth.

Possible Answers:

\(\displaystyle 2.1\)

\(\displaystyle 2.9\)

\(\displaystyle 2.3\)

\(\displaystyle 2.5\)

\(\displaystyle 2.7\)

Correct answer:

\(\displaystyle 2.5\)

Explanation:

Since most calculators only have common and natural logarithm keys, this can best be solved as follows:

By the Change of Base Property of Logarithms, if \(\displaystyle M,N,a > 0\) and \(\displaystyle a, M \ne 1\)

\(\displaystyle \log_{M}N = \frac{\log_{a}N}{\log_{a}M}\)

Setting \(\displaystyle M = 8, N = 172, a = 10\), we can restate this logarithm as the quotient of two common logarithms, and calculate accordingly:

\(\displaystyle \log_{8}172 = \frac{\log 172}{\log 8}\approx \frac{2.2355 }{0.9031}\approx 2.4754\)

or, when rounded, 2.5. 

This can also be done with natural logarithms, yielding the same result.

\(\displaystyle \log_{8}172 = \frac{\ln 172}{\ln 8} \approx \frac{5.1475}{2.0794}\approx 2.4754\)

Rond to one decimal place.

\(\displaystyle 2.5\)

Example Question #41 : Understanding Logarithms

Solve for \(\displaystyle x\) in the equation:

\(\displaystyle -log(x)=4.2\)

Possible Answers:

\(\displaystyle x=e^{-4.2}\)

\(\displaystyle x=10^{-4.2}\)

\(\displaystyle x=10^{-log(4.2)}\)

\(\displaystyle x=e^{4.2}\)

Correct answer:

\(\displaystyle x=10^{-4.2}\)

Explanation:

This question tests your understanding of log functions.

\(\displaystyle log(x)=y\) can be converted to the form \(\displaystyle x=10^{y}\).

In this problem, make sure to divide both sides by \(\displaystyle -1\) in order to put it in the above form, where \(\displaystyle log(x)=-4.2\). Remember \(\displaystyle log(x)=log_{10}(x)\).

Therefore,

\(\displaystyle log_{10}(x)=-4.2 \rightarrow x=10^{-4.2}\)

Example Question #281 : Mathematical Relationships And Basic Graphs

Evaluate \(\displaystyle x\).

\(\displaystyle 5 ^{x} = 12\)

Possible Answers:

\(\displaystyle x= \frac{ \log 5}{12}\)

\(\displaystyle x= \frac{ \log 12 }{5}\)

\(\displaystyle x= \log 7\)

\(\displaystyle x=\frac{ \log 5}{\log 12}\)

\(\displaystyle x=\frac{\log 12}{\log 5}\)

Correct answer:

\(\displaystyle x=\frac{\log 12}{\log 5}\)

Explanation:

Take the common logarithm of both sides, and take advantage of the property of the logarithm of a power:

\(\displaystyle 5 ^{x} = 12\)

\(\displaystyle \log 5 ^{x} = \log 12\)

\(\displaystyle x \log 5 = \log 12\)

\(\displaystyle \frac{x \log 5 }{\log 5 }=\frac{ \log 12}{\log 5}\)

\(\displaystyle x=\frac{ \log 12}{\log 5}\)

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