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3.4.2: Common and Natural Logarithms

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    14377
  • Common and Natural Logarithms

    By now, you know that log264=x can be solved if you recognize that 26=64. What about numbers that aren't so 'clean'? There aren't many people who could calculate the answer to log7247=x in their head! It would be great to use a calculator, but most only have two log functions: base 10 and base e.

    Is there a way to convert from one base to another, so we can use a calculator?


    Common and Natural Logarithms

    Although a log function can have any positive number as a base, there are really only two bases that are commonly used in the real world. Both may be written without a base noted, like: logx, so you may need to use the context to decide which is appropriate.

    The common log is a log with base 10. It is used to define pH, earthquake magnitude, and sound decibel levels, among many many other common real-world values.

    The natural log, sometimes written ln(x), is a log with base e. The transcendental number e is approximately 2.71828 and is used in any number of calculations involving constant growth in chemistry, physics, biology, finance, etc..

    Using a Calculator for logs

    You may have noticed that the common log and the natural log are the only log buttons on your calculator. We can use either the common log or the natural log to find the values of logs with other bases.

    The equation \(\ \log _{b} x=\frac{\log x}{\log b}\) is called the change of base formula, and may be used to convert to common log or natural log.

    You may also see the change of base formula as \(\ \log _{b} x=\frac{\ln x}{\ln b}\), which is the same formula specifying a conversion to the natural log.

    Using the change of base formula, we can find the common log (or the natural log) equivalent of any other base so that we can use a calculator to find the value of an expression.

    Consider log3 35. If we use the change of base formula to convert to base 10, and then the log button on a calculator, we find that \(\ \log _{3} 35=\frac{\log 35}{\log 3}=3.23621727\). 

     


    Examples

    Example 1

    Earlier, you were asked to solve the following problem: \(\ \log _{7} 247=x\)

    Solution

    Using the change of base formula: \(\ \log _{7} 247=\frac{\log 247}{\log 7}\)

    Using a calculator to find the common logs of 247 and 7, we get (approximately) \(\ \frac{2.4}{.8}=2.8313\).

    We can verify with \(\ 7^{2.8313}=247\)

    \(\ \therefore \log _{7} 247=2.8313\)

    Example 2

    Evaluate each log.

    Remember that logx (with no base specified) commonly refers to log10x.

    1. \(\ \log 1\)
    2. \(\ \log 10\)
    3. \(\ \log \sqrt{10}\)

    Solution

    1. \(\ \log 1=0 \text { because } 10^{0}=1\)
    2. \(\ \log 10=1 \text { because } 10^{1}=10\)
    3. \(\ \log \sqrt{10}=\frac{1}{2} \text { because } \sqrt{10}=10^{1 / 2}\)

    Example 3

    For each log value, determine two integers between which the log value should lie. Then use a calculator to find the value of the log.

    1. \(\ \log 50\)
    2. \(\ \log 818\)

    Solution

    1. The value of this log should be between 1 and 2, as 101 = 10, and 102 = 100.

      Using a calculator, you should find that log 50 ≈ 1.698970004.

    2. The value of this log should be between 2 and 3, as 102 = 100, and 103 = 1000.

      Using a calculator, you should find that log 818 ≈ 2.912753304.

    Example 4

    Estimate the value, and then use the change of base formula to find the value of \(\ \log _{2} 17\).

    Solution

    \(\ \log _{2} 17\) is close to 4 because 24=16 and 25=32.

    Using the change of base formula, we have \(\ \log _{2} 17=\frac{\log 17}{\log 2}\).

    Using a calculator, you should find that the approximate value of this expression is 4.087462841.

    Example 5

    Find the value of each natural log.

    1. \(\ \ln 100\)
    2. \(\ \ln \sqrt{e}\)

    Solution

    1. \(\ \ln 100\) is between 4 and 5. You can estimate this by rounding e up to 3, and considering powers of 3:

      3^{4}=81 \text { and } 3^{5}=243 

      Using a calculator, you should find that \(\ \ln 100=4.605171086\)

    2. Recall that a square root is the same as an exponent of 1/2.

      Therefore \(\ \ln \sqrt{e}=\ln \left(e^{1 / 2}\right)=1 / 2\)

    Example 6

    Solve the equation: \(\ 5^{x}=3 \cdot 7^{x}\)

    Solution

    To solve: \(\ 3^{x}\left(2^{3 x}\right)=7\left(5^{x}\right)\)

    \(\ 3^{x}\left(2^{3}\right)^{x}=7\left(5^{x}\right)\): Rule of exponents \(\ \left(x^{y}\right)^{z}=x^{y z}\)

    \(\ 3^{x}\left(8^{x}\right)=7\left(5^{x}\right) \rightarrow 24^{x}=7\left(5^{x}\right)\): By multiplication

    \(\ \left(\frac{24}{5}\right)^{x}=7\): Divide both sides by \(\ 5^x\)

    \(\ \log \left(\frac{24}{5}\right)^{x}=\log 7\): Take the log of both sides

    \(\ x \log \left(\frac{24}{5}\right)=\log 7\): Using \(\ \log x^{y}=y \log x\)

    \(\ x=\frac{\log 7}{\log \frac{24}{5}}\): Divide both sides by \(\ \log \left(\frac{24}{5}\right)\)

    \(\ x=1.24\): With a calculator

    Example 7

    Find the value: \(\ \ln 6+\ln 7\)

    Solution

    Use a calculator to find the values:

    \(\ \ln 6=1.79175 \text { and } \ln 7=1.94591\)

    1.79175 + 1.94591 = 3.73766


    Review

    1. What is a common logarithm? Where are common logs most commonly used?
    2. What is a natural logarithm? Where are natural logs commonly used?

    Evaluate each expression:

    1. \(\ \log \frac{17^{4}}{5}\)
    2. \(\ \log 7\left(4^{3}\right)\)

    Convert to a common logarithm and evaluate:

    1. \(\ \log _{6} 832\) 
    2. \(\ \log _{11} 47\) 
    3. \(\ \log _{3} 9\)

    Convert to a natural logarithm and evaluate:

    1. \(\ \log _{7} 91\) 
    2. \(\ \log_5256\)
    3. \(\ \log_90.712\)

    Find the values of the natural logarithms:

    1. \(\ \ln56\)
    2. \(\ \ln2000\)
    3. \(\ \ln950.1\)
    4. \(\ \ln.9\)

    Convert the natural logs to exponential form, and solve.

    1. If \(\ \text { lne }=x \text { and } e^{x}=e \text { then } x=?\)
    2. If \(\ \ln e^{5} \text { then } x=?\)
    3. If \(\ \ln e^{a}=x \text { then } x=?\)
    4. If \(\ \ln e^{-3}=x \text { then } x=?\)

    Review (Answers)

    To see the Review answers, open this PDF file and look for section 3.8.


    Vocabulary

    Term Definition
    e e is an irrational number that is approximately equal to 2.71828. As \(\ n \rightarrow \infty,\left(1+\frac{1}{n}\right)^{n} \rightarrow e\).
    Change of Base Formula Let b, x, and y be positive numbers, b≠1 and y≠1. Then, \(\ \log _{y} x=\frac{\log _{b} x}{\log _{b} y}\). More specifically, \(\ \log _{y} x=\frac{\log x}{\log y}\) and \(\ \log _{y} x=\frac{\ln x}{\ln y}\), so that expressions can be evaluated using a calculator.
    Common Log A common logarithm is a log with base 10. The log is usually written without the base.
    Common Logarithm A common logarithm is a log with base 10. The log is usually written without the base.
    e e is an irrational number that is approximately equal to 2.71828. As \(\ n \rightarrow \infty,\left(1+\frac{1}{n}\right)^{n} \rightarrow e\).
    Natural Log A natural logarithm is a log with base e. The natural logarithm is written as ln.
    Natural Logarithm A natural logarithm is a log with base e. The natural logarithm is written as ln.
    Transcendental Number A transcendental number is a number that is not the root of any rational polynomial function. Examples include e and π.
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