Learning Outcomes
- Expand a logarithmusing a combination of logarithm rules.
- Condense a logarithmic expression into one logarithm.
Expanding Logarithms
Taken together, the product rule, quotient rule, and power rule are often called “properties of logs.” Sometimes we apply more than one rule in order to expand an expression. For example:
[latex]\begin{array}{l}{\mathrm{log}}_{b}\left(\frac{6x}{y}\right)\hfill & ={\mathrm{log}}_{b}\left(6x\right)-{\mathrm{log}}_{b}y\hfill \\ \hfill & ={\mathrm{log}}_{b}6+{\mathrm{log}}_{b}x-{\mathrm{log}}_{b}y\hfill \end{array}[/latex]
We can use the power rule to expand logarithmic expressions involving negative and fractional exponents. Here is an alternate proof of the quotient rule for logarithms using the fact that a reciprocal is a negative power:
[latex]\begin{array}{l}{\mathrm{log}}_{b}\left(\frac{A}{C}\right)\hfill & ={\mathrm{log}}_{b}\left(A{C}^{-1}\right)\hfill \\ \hfill & ={\mathrm{log}}_{b}\left(A\right)+{\mathrm{log}}_{b}\left({C}^{-1}\right)\hfill \\ \hfill & ={\mathrm{log}}_{b}A+\left(-1\right){\mathrm{log}}_{b}C\hfill \\ \hfill & ={\mathrm{log}}_{b}A-{\mathrm{log}}_{b}C\hfill \end{array}[/latex]
We can also apply the product rule to express a sum or difference of logarithms as the logarithm of a product.
With practice, we can look at a logarithmic expression and expand it mentally and then just writing the final answer. Remember, however, that we can only do this with products, quotients, powers, and roots—never with addition or subtraction inside the argument of the logarithm.
Example: Using a combination of the rules for logarithms to expand a logarithm
Rewrite [latex]\mathrm{ln}\left(\frac{{x}^{4}y}{7}\right)[/latex] as a sum or difference of logs.
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Expand [latex]\mathrm{log}\left(\frac{{x}^{2}{y}^{3}}{{z}^{4}}\right)[/latex].
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In the next example we will recall that we can write roots as exponents, and use this quality to simplify logarithmic expressions.
Example: Using the Power Rule for Logarithms to Simplify the Logarithm of a Radical Expression
Expand [latex]\mathrm{log}\left(\sqrt{x}\right)[/latex].
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Expand [latex]\mathrm{ln}\left(\sqrt[3]{{x}^{2}}\right)[/latex].
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Q & A
Can we expand [latex]\mathrm{ln}\left({x}^{2}+{y}^{2}\right)[/latex]?
No. There is no way to expand the logarithm of a sum or difference inside the argument of the logarithm.
Now we will providesome examples that will require careful attention.
Example: Expanding Complex Logarithmic Expressions
Expand [latex]{\mathrm{log}}_{6}\left(\frac{64{x}^{3}\left(4x+1\right)}{\left(2x - 1\right)}\right)[/latex].
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Expand [latex]\mathrm{ln}\left(\frac{\sqrt{\left(x - 1\right){\left(2x+1\right)}^{2}}}{\left({x}^{2}-9\right)}\right)[/latex].
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Condensing Logarithms
We can use the rules of logarithms we just learned to condense sums, differences, and products with the same base as a single logarithm. It is important to remember that the logarithms must have the same base to be combined. We will learn later how to change the base of any logarithm before condensing.
How To: Given a sum, difference, or product of logarithms with the same base, write an equivalent expression as a single logarithm
- Apply the power property first. Identify terms that are products of factors and a logarithm and rewrite each as the logarithm of a power.
- From left to right, apply the product and quotient properties. Rewrite sums of logarithms as the logarithm of a product and differences of logarithms as the logarithm of a quotient.
Example: Using the Power Rule in Reverse
Use the power rule for logs to rewrite [latex]4\mathrm{ln}\left(x\right)[/latex] as a single logarithm with a leading coefficient of 1.
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Use the power rule for logs to rewrite [latex]2{\mathrm{log}}_{3}4[/latex] as a single logarithm with a leading coefficient of 1.
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In our next few examples we will use a combination of logarithm rules to condense logarithms.
Example: Using the Product and Quotient Rules to Combine Logarithms
Write [latex]{\mathrm{log}}_{3}\left(5\right)+{\mathrm{log}}_{3}\left(8\right)-{\mathrm{log}}_{3}\left(2\right)[/latex] as a single logarithm.
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Condense [latex]\mathrm{log}3-\mathrm{log}4+\mathrm{log}5-\mathrm{log}6[/latex].
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Example: Condensing Complex Logarithmic Expressions
Condense [latex]{\mathrm{log}}_{2}\left({x}^{2}\right)+\frac{1}{2}{\mathrm{log}}_{2}\left(x - 1\right)-3{\mathrm{log}}_{2}\left({\left(x+3\right)}^{2}\right)[/latex].
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Example: Rewriting as a Single Logarithm
Rewrite [latex]2\mathrm{log}x - 4\mathrm{log}\left(x+5\right)+\frac{1}{x}\mathrm{log}\left(3x+5\right)[/latex] as a single logarithm.
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Rewrite [latex]\mathrm{log}\left(5\right)+0.5\mathrm{log}\left(x\right)-\mathrm{log}\left(7x - 1\right)+3\mathrm{log}\left(x - 1\right)[/latex] as a single logarithm.
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Condense [latex]4\left(3\mathrm{log}\left(x\right)+\mathrm{log}\left(x+5\right)-\mathrm{log}\left(2x+3\right)\right)[/latex].
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Applications of Properties of Logarithms
In chemistry, pH is a measure of how acidic or basic a liquid is. It is essentially a measure of the concentration of hydrogen ions in a solution. The scale for measuring pH is standardized across the world, the scientific community having agreed upon its values and methods for acquiring them.
Measurements of pH can help scientists, farmers, doctors, and engineers solve problems and identify sources of problems.
pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, [latex]a_{H}+[/latex], in a solution.
[latex]\text{pH} =-\log _{10}(a_{{\text{H}}^{+}})=\log _{10}\left({\frac {1}{a_{{\text{H}}^{+}}}}\right)[/latex]
For example, a solution with a hydrogen ion activity of [latex]2.5×{10}^{-6}[/latex] (at that level essentially the number of moles of hydrogen ions per liter of solution) has a pH of [latex]\log_{10}\left(\frac{1}{2.5×{10}^{-6}}\right)=5.6[/latex]
In the next examples, we will solve some problems involving pH.
Example: Applying Properties of Logs
Recall that, in chemistry, [latex]\text{pH}=-\mathrm{log}\left[{H}^{+}\right][/latex]. If the concentration of hydrogen ions in a liquid is doubled, what is the effect on pH?
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How does the pH change when the concentration of positive hydrogen ions is decreased by half?
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