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  1. Home
  2. 12th grade
  3. Precalculus
  4. Exercise : Convert between a difference of logarithms and a quotient

Convert between a difference of logarithms and a quotient Precalculus

Change the following logarithmic expressions.

\log_2\left(3x\right)-\log_2\left(6x\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(x\right)-\log_a\left(y\right)=\log_a\left(\dfrac{x}{y}\right)

Applying this formula to our problem:

\log_2\left(3x\right)-\log_2\left(6x\right)=\log_2\left(\dfrac{3x}{6x}\right)=\log_2\left(\dfrac{3}{6}\right)=\log_2\left(\dfrac{1}{2}\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem:

2^{c}=\dfrac{1}{2}

Since:

\dfrac{1}{2}=2^{-1}

We conclude that:

\log_2\left(\dfrac{1}{2}\right)=-1

\log_2\left(3x\right)-\log_2\left(6x\right)=-1

\log_3\left(9x^{2}\right)-\log_3\left(3x^{2}\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(x\right)-\log_a\left(y\right)=\log_a\left(\dfrac{x}{y}\right)

Applying this formula to our problem:

\log_3\left(9x^{2}\right)-\log_3\left(3x^{2}\right)=\log_3\left(\dfrac{9x^{2}}{3x^{2}}\right)=\log_3\left(3\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem:

3^{c}=3

Since:

3=3^{1}

We conclude that:

\log_3\left(3\right)=1

\log_3\left(9x^{2}\right)-\log_3\left(3x^{2}\right)=1

\log_5\left(\sqrt[3]{625 m^{2}}\right)-\log_5\left(\sqrt[3]{25 m^{2}}\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(x\right)-\log_a\left(y\right)=\log_a\left(\dfrac{x}{y}\right)

Applying this formula to our problem:

\log_5\left(\sqrt[3]{625 m^{2}}\right)-\log_5\left(\sqrt[3]{25 m^{2}}\right)=\log_5\left(\dfrac{\sqrt[3]{625 m^{2}}}{\sqrt[3]{25 m^{2}}}\right)=\log_5\left(\sqrt[3]{25}\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem:

5^{c}=\sqrt[3]{25}

Since:

\sqrt[3]{25}=5^{\frac{2}{3}}

We conclude that:

\log_5\left(\sqrt[3]{25}\right)=\dfrac{2}{3}

\log_5\left(\sqrt[3]{625 m^{2}}\right)-\log_5\left(\sqrt[3]{25 m^{2}}\right)=\dfrac{2}{3}

\ln\left(x^{2}\right)-\ln\left(ex^{2}\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(x\right)-\log_a\left(y\right)=\log_a\left(\dfrac{x}{y}\right)

Applying this formula to our problem:

\ln\left(x^{2}\right)-\ln\left(ex^{2}\right)=\ln\left(\dfrac{x^{2}}{ex^{2}}\right)=\ln\left(\dfrac{1}{e}\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem:

e^{c}=\dfrac{1}{e}

Since:

\dfrac{1}{e}=e^{-1}

We conclude that:

\ln\left(\dfrac{1}{e}\right)=-1

\ln\left(x^{2}\right)-\ln\left(ex^{2}\right)=-1

\log_2\left(\dfrac{8}{7}\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(\dfrac{x}{y}\right)=\log_a\left(x\right)-\log_a\left(y\right)

Applying this formula to our problem:

\log_2\left(\dfrac{8}{7}\right)=\log_2\left(8\right)-\log_2\left(7\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem, when calculating \log_2\left(8\right) :

2^{c}=8

Since:

8=2^{3}

We conclude that:

\log_2\left(8\right)=3

\log_2\left(\dfrac{8}{7}\right)=3-\log_2\left(7\right)

\ln\left(\dfrac{3}{e^{2}}\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(\dfrac{x}{y}\right)=\log_a\left(x\right)-\log_a\left(y\right)

Applying this formula to our problem:

\ln\left(\dfrac{3}{e^{2}}\right)=\ln\left(3\right)-\ln\left(e^{2}\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem, when calculating \ln\left(e^{2}\right) :

e^{c}=e^{2}

Since:

e^{2}=e^{2}

We conclude that:

\ln\left({e^{2}}\right)=2

\ln\left(\dfrac{3}{e^{2}}\right)=\ln\left(3\right)-2

\log\left(\dfrac{\sqrt[4]{100}}{7}\right)

The formula for a difference of logarithms with the same base is:

\log_a\left(\dfrac{x}{y}\right)=\log_a\left(x\right)-\log_a\left(y\right)

Applying this formula to our problem:

\log\left(\dfrac{\sqrt[4]{100}}{7}\right)=\log\left(\sqrt[4]{100}\right)-\log\left(7\right)

When calculating \log_a\left(b\right), we are looking for a real number c so that:

a^{c}=b

In our problem, when calculating \log\left(\sqrt[4]{100}\right) :

10^{c}=\sqrt[4]{100}

Since:

\sqrt[4]{100}=10^{\frac{2}{4}}=10^{\frac{1}{2}}

We conclude that:

\log\left(\sqrt[4]{100}\right)=\dfrac{1}{2}

\log\left(\dfrac{\sqrt[4]{100}}{7}\right)=\dfrac{1}{2}-\log\left(7\right)

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See also
  • Course : Logarithmic functions
  • Exercise : Determine the domain and range of logarithms from equations of functions
  • Exercise : Match logarithmic functions and graphs
  • Exercise : Convert between a sum of logarithms and a product
  • Exercise : Convert between a multi-term sum of logarithms and a power
  • Exercise : Change the base of a logarithm
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