Algebra II : Understanding Logarithms

Study concepts, example questions & explanations for Algebra II

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Example Questions

Example Question #21 : Natural Log

Solve:  \displaystyle ln(2x-e) = 3

Possible Answers:

\displaystyle \frac{e^3+e}{2}

\displaystyle \frac{(e+1)^3}{2}

\displaystyle \frac{e^3-e}{2}

\displaystyle \frac{3}{2}e

\displaystyle \frac{9-e}{3}

Correct answer:

\displaystyle \frac{e^3+e}{2}

Explanation:

In order to eliminate the natural log, which has a base of \displaystyle e, we will need to raise both side as powers of \displaystyle e.

\displaystyle e^{ln(2x-e)} = e^3

The equation can be simplified to:

\displaystyle 2x-e = e^3

Add \displaystyle e on both sides.

\displaystyle 2x-e +e= e^3+e

\displaystyle 2x= e^3+e

Divide by two on both sides.

\displaystyle \frac{2x}{2}= \frac{e^3+e}{2}

The answer is:  \displaystyle \frac{e^3+e}{2}

Example Question #261 : Mathematical Relationships And Basic Graphs

Try without a calculator:

Which expression is not equivalent to 1?

Possible Answers:

\displaystyle 1,000,000^{0}

\displaystyle \log 10

\displaystyle i^{4}

\displaystyle 1,000^{\frac{1}{1,000}}

\displaystyle \ln e

Correct answer:

\displaystyle 1,000^{\frac{1}{1,000}}

Explanation:

\displaystyle 1,000^{\frac{1}{1,000}} is the correct choice.

For all \displaystyle a,N for which the expressions are defined, 

\displaystyle a ^{\frac{1}{N}} = \sqrt[N]{a}.

Setting \displaystyle a=1,000, N = 1,000, this equation becomes

\displaystyle 1,000 ^{\frac{1}{1,000}} = \sqrt[1,000]{1,000} - that is, the one thousandth root of 1,000. This is not equal to 1, since if it were, it would hold that \displaystyle 1^{1,000} = 1,000 - which is not true. 

 

Of the other four expressions:

\displaystyle \log 10, the common, or base ten, logarithm of 10, can be rewritten as \displaystyle \log_{10}10, and \displaystyle \ln e, the natural, or base \displaystyle e, logarithm of \displaystyle e, can be rewritten as \displaystyle \log_{e}e. A property of logarithms states that for all \displaystyle N > 0, N \ne 1\displaystyle \log_{N}N = 1. Therefore, \displaystyle \log 10 = \log_{10}10 = 1 and  \displaystyle \ln e = \log_{e}e = 1.

 

\displaystyle 1,000,000^{0} = 1, since any nonzero number raised to the power of 0 is equal to 1.

 

By the Power of a Power Property, 

\displaystyle i^{4} = i ^{2 \cdot 2} =( i^{2} )^{2}

\displaystyle i^{2} = -1, so

\displaystyle i^{4} = ( -1)^{2}= 1

Example Question #1 : Log Base 10

Based on the definition of logarithms, what is \displaystyle {\log_{10}1000} ? 

Possible Answers:

4

10

3

100

2

Correct answer:

3

Explanation:

For any equation \displaystyle \log_{10}(y) = x, \displaystyle 10^{x } = y. Thus, we are trying to determine what power of 10 is 1000. \displaystyle 1000 = 10^3, so our answer is 3. 

Example Question #2 : Log Base 10

\displaystyle 7 ^{x} = 1,000

Evaluate \displaystyle x.

Possible Answers:

\displaystyle \log 993

\displaystyle x = 1,000 - \log 7

\displaystyle x = 3 - \log 7

\displaystyle x = \frac{1,000}{\log 7}

\displaystyle x = \frac{3}{\log 7}

Correct answer:

\displaystyle x = \frac{3}{\log 7}

Explanation:

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

\displaystyle 7 ^{x} = 1,000

\displaystyle 7 ^{x} = 10^{3}

\displaystyle \log 7 ^{x} = \log 10^{3}

\displaystyle x \log 7 = 3 \log 10

\displaystyle x \log 7 = 3 \cdot 1

\displaystyle x \log 7 = 3

\displaystyle \frac{x \log 7}{\log 7} = \frac{3}{\log 7}

\displaystyle x = \frac{3}{\log 7}

Example Question #2 : Log Base 10

What is the value of \displaystyle \log{10000000}?

Possible Answers:

\displaystyle 7.31

\displaystyle 7

\displaystyle 5.68

\displaystyle 6.97

\displaystyle 8.13

Correct answer:

\displaystyle 7

Explanation:

Base-10 logarithms are very easy if the operands are a power of \displaystyle 10.  Begin by rewriting the question:

\displaystyle \log{10000000}

Becomes...

\displaystyle \log{10^7}

because \displaystyle 10^7=10000000

Applying logarithm rules, you can factor out the \displaystyle 7:

\displaystyle 7\log{10}

Now, \displaystyle \log{10} is \displaystyle 1.

Therefore, your answer is \displaystyle 7.

Example Question #3 : Log Base 10

What is the value of \displaystyle \log{100000}?

Round to the nearest hundreth.

Possible Answers:

\displaystyle 5.10

\displaystyle 4.52

\displaystyle 5

\displaystyle 6

\displaystyle 5.01

Correct answer:

\displaystyle 5

Explanation:

Base-10 logarithms are very easy if the operands are a power of \displaystyle 10.  Begin by rewriting the question:

\displaystyle \log{100000}

Becomes...

\displaystyle \log{10^5}

because \displaystyle 10^5=100000

Applying logarithm rules, you can factor out the \displaystyle 5:

\displaystyle 5\log{10}

Now, \displaystyle \log{10} is \displaystyle 1.

Therefore, your answer is \displaystyle 5.

Example Question #2 : Log Base 10

Many textbooks use the following convention for logarithms: 

\displaystyle log(x) = log_{10}(x)

\displaystyle ln(x) = log_e(x)

\displaystyle lg(x)=log_2(x)

What is the value of \displaystyle log1,000?

Possible Answers:

\displaystyle 2

\displaystyle -2

\displaystyle 4

\displaystyle 100

\displaystyle 3

Correct answer:

\displaystyle 3

Explanation:

Remember:

\displaystyle log_ab=c is the same as saying \displaystyle a^c = b.

So when we ask "What is the value of \displaystyle log(1,000)?", all we're asking is "10 raised to which power equals 1,000?" Or, in an expression: 

\displaystyle 10^? = 1,000.

From this, it should be easy to see that \displaystyle log(1000)=3.

Example Question #1 : Log Base 10

Evaluate the following expression:

 

\displaystyle \small \log (1000)

Possible Answers:

\displaystyle \small 10

\displaystyle \small 1

\displaystyle \small 5

\displaystyle \small 3

\displaystyle \small 2

Correct answer:

\displaystyle \small 3

Explanation:

Without a subscript a logarithmic expression is base 10.

The expression \displaystyle \small \log(1000)=\log _{10}(1000) 

The logarithmic expression is asking 10 raised to what power equals 1000 or what is x when

\displaystyle \small 10^{x}=1000

We know that \displaystyle \small 10^{3}=1000

so \displaystyle \small \log (1000)=3

Example Question #7 : Log Base 10

Assuming the value of \displaystyle x is positive, simplify:  \displaystyle \log_{10}10^{3x+2}

Possible Answers:

\displaystyle -3x-2

\displaystyle 1

\displaystyle 30x+20

\displaystyle -30x-20

\displaystyle 3x+2

Correct answer:

\displaystyle 3x+2

Explanation:

Rewrite the logarithm in division.

\displaystyle \log_{10}10^{3x+2} = \frac{\log{10}^{3x+2}}{\log 10}

As a log property, we can pull down the exponent of the power in front as the coefficient.

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

Cancel out the \displaystyle \log(10).

The answer is:  \displaystyle 3x+2

Example Question #8 : Log Base 10

Solve the following:

\displaystyle \log10000+\log10

Possible Answers:

\displaystyle 4

\displaystyle \log10010

\displaystyle 5

\displaystyle 3

Correct answer:

\displaystyle 5

Explanation:

When the base isn't explicitly defined, the log is base 10. For our problem, the first term

\displaystyle \log 10000=x

is asking:

\displaystyle 10^x=10000

\displaystyle x=4

For the second term,

\displaystyle \log(10)=y

is asking:

\displaystyle 10^y=10

\displaystyle y=1

So, our final answer is \displaystyle 4+1=5

 

 

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