Laws of Logarithms

The Laws of Logarithms are mathematical rules that simplify the calculation of logarithms, involving products, quotients, powers, and roots. Understanding these laws is essential for solving logarithmic equations and working with logarithmic expressions.

Below is a list of all the exponent rules that will come in handy when encountering and simplifying different types of logarithmic expressions:


Name Rule
Zero \(\log_a 1 = 0\)
Identity \(\log_a a = 1\)
Inverse Property of Logarithm \(\log_a(a^k) = k\)
Product \(\log_a(x \cdot y) = \log_a x + \log_a y\)
Quotient \(\log_a\left(\cfrac{x}{y}\right) = \log_a x - \log_a y \)
Power \(\log_a(x^n) = n \log_a x\)
Change of Base \(\log_a x = \cfrac{\log_b x}{\log_b a} \)
Equality \(\log_ax = \log_ay \rightarrow x = y\)

Expand the following by using Laws of Logarithms.

\(\log_a (x^3y) \)

First, we can use Product Rule, \(\log_a(x \cdot y) = \log_a x + \log_a y\):

\(\log_a (x^3y) = \log_a (x^3) + \log_a (y)\)

Next, we can use Power Rule, \(\log_a(x^n) = n \log_a x\):

\( \log_a (x^3y) = 3\log_a (x) - \log_a (y) \)

Therefore, we can determine that \( \log_a (x^3y)\) expanded is \(\boldsymbol{3\log_a (x) - \log_a (y)}\).


\(\log_b (x^2y^3)\)

First, we can use Product Rule, \(\log_a(x \cdot y) = \log_a x + \log_a y\):

\( \log_b (x^2y^3) = \log_b (x^2) + \log_b (y^3) \)

Next, we can use Power Rule, \(\log_a(x^n) = n \log_a x\):

\( \log_b (x^2y^3) = 2\log_b (x) - 3\log_b (y) \)

Therefore, we can determine that \(\log_b (x^2y^3)\) expanded is \(\boldsymbol{2\log_b (x) - 3\log_b (y)}\).


\(\log_c \left(\cfrac{1}{x^4 y^3} \right)\)

First, we can use the Quotient Rule, \(\log_a \left(\cfrac{x}{y}\right) = \log_a x - \log_a y \):

\(\log_c \left( \cfrac{1}{x^4y^3} \right) = \log_c (1) - \log_c (x^4y^3)\)

Next, we can use the Zero Rule, \(\log_a 1 = 0\):

\(\log_c \left( \cfrac{1}{x^4y^3} \right) = - \log_c (x^4y^3)\)

Then, we can use Product Rule, \(\log_a(x \cdot y) = \log_a x + \log_a y\):

\(\log_c \left( \cfrac{1}{x^4y^3} \right) = -4\log_c (x) - 3\log_c (y)\)

Therefore, we can determine that \(\log_c \left(\cfrac{1}{x^4 y^3} \right)\) expanded is \(\boldsymbol{-4\log_c (x) - 3\log_c (y)}\).


\(\log_d \left(\cfrac{x^5}{y^{1/3}} \right)\)

First, we can use the Quotient Rule, \(\log_a\left(\cfrac{x}{y}\right) = \log_a x - \log_a y \):

\(\log_d \left( \cfrac{x^5}{y^{1/3}} \right) = \log_d (x^5) - \log_d (y^{1/3})\)

Next, we can use the Power Rule, \(\log_a(x^n) = n \log_a x\):

\(\log_d \left( \cfrac{x^5}{y^{1/3}} \right) = 5\log_d (x) - \cfrac{1}{3}\log_d (y)\)

Therefore, we can determine that \(\log_d \left(\cfrac{x^5}{y^{1/3}} \right)\) expanded is \(\boldsymbol{5\log_d (x) - \cfrac{1}{3}\log_d (y)}\).


Condense the following into a single logarithm.

\(\cfrac{1}{3}\log_e (x^3) - \cfrac{1}{2}\log_e (y^6)\)

First, we can split \(y^6\) using the Power of a Product, \(a^{mn} = (a^m)^n\):

\(\cfrac{1}{3} \cdot \log_e(x^3) - \cfrac{1}{2} \cdot \log_e((y^3)^2)\)


Next, we can use Power Rule, \(\log_a(x^n) = n \log_a x\), to the logarithms:

\(\cfrac{1}{3} \cdot 3\log_e(x) - \cfrac{1}{2} \cdot 2\log_e(y^3)\)

Then, we can simplify the expression by multiplying the coefficients:

\(\log_e(x) - \log_e(y^3)\)

After, we can use Quotient Rule, \( \log_a(\cfrac{x}{y}) = \log_a x - \log_a y\) to combine the logarithms:

\(\log_e\left(\cfrac{x}{y^3}\right)\)

Therefore, we can determine that \( \cfrac{1}{3}\log_e (x^3) - \cfrac{1}{2}\log_e (y^6) \) condensed is \(\boldsymbol{\log_e\left(\cfrac{x}{y^3}\right)}\).


\(\cfrac{2}{3}\log_f (x^3 y^2) - \cfrac{1}{2}\log_f (x^4 y^3)\)

First, we can split \((x^3 y^2)\) and \((x^4 y^3)\) using the Power of a Product, \(a^{mn} = (a^m)^n\):

\(\cfrac{2}{3}\log_f ((x^2 y^{4/3})^{3/2}) - \cfrac{1}{2}\log_f ((x^2 y^{3/2})^2)\)

Next, we can use Power Rule, \(\log_a(x^n) = n \log_a x\):

\(\cfrac{2}{3} \cdot \cfrac{3}{2}\log_f(x^3y^2) - \cfrac{1}{2} \cdot 2\log_f(x^4y^3)\)

Then, we can simplify the expression by multiplying the coefficients:

\(\log_f(x^2y) - \log_f(x^2y^{3/2})\)

After, we can use the Quotient Rule, \( \log_a(\cfrac{x}{y}) = \log_a x - \log_a y\) to combine the logarithms:

\(\log_f\left(\cfrac{x^2y}{x^2y^{3/2}}\right)\)

Finally, we can simplify the expression inside the logarithm by cancelling powers with the same base:

\(\log_f(y^{-1/6})\)

Therefore, we can determine that \( \cfrac{2}{3}\log_f (x^3 y^2) - \cfrac{1}{2}\log_f (x^4 y^3) \) condensed is \(\boldsymbol{\log_f(y^{-1/6})}\).


\(3\log_g (x) + \cfrac{1}{2}\log_g (y)\)

First, we can use the Power Rule, \(n \log_a x = \log_a(x^n)\), on the logarithms:

\(\log_g(x^3) + \log_g(y^{1/2})\)

Next, we can combine the logarithms using Product Rule, \(\log_a(x \cdot y) = \log_a x + \log_a y\):

\(\log_g(x^3y^{1/2})\)

Therefore, we can determine that \(3\log_g (x) + \cfrac{1}{2}\log_g (y) \) condensed is \(\boldsymbol{\log_g(x^3y^{1/2})}\).


\(3\log_h (x) - \cfrac{2}{3}\log_h (y^3)\)

First, we can use the Power Rule, \(\log_a(x^n) = n \log_a x\), on the logarithms:

First, we can use the Power Rule, \(n \log_a x = \log_a(x^n)\), on the logarithms:

\(\log_h(x^3) - \log_h(y^2)\)

Next, we can combine the logarithms using Quotient Rule, \(\log_a\left(\cfrac{x}{y}\right) = \log_a x - \log_a y\):

\(\log_h\left(\cfrac{x^3}{y^2}\right)\)

Therefore, we can determine that \(3\log_h (x) - \cfrac{2}{3}\log_h (y^3)\) condensed is \(\boldsymbol{\log_h\left(\cfrac{x^3}{y^2}\right)}\).


Evaluate the following expressions.

\(\log_3 (27^2)\)

First, we can apply the exponent to the term inside the logarithm:

\(\log_3 (27^2) =\log_3 (729)\)

Next, we can identify the power the exponential must be raised to to obtain \(729\) with a base of \(3\):

\(\log_3 (27^2) = \log_3 (3^6)\)

Then, we can use the Inverse Property of Logarithm since the base of the exponential is the same as the base of the logarithm:

\(\log_3 (27^2) = 6\)

Therefore, we can evaluate \(\log_3 (27^2)\) as equal to \(\boldsymbol{6}\).


\(\log_2 (32)\)

First, we can identify the power to which the exponential must be raised to obtain \(32\) with a base of \(3\):

\(\log_2 (32) = \log_2 (2^5) \)

Next, we can use the Inverse Property of Logarithm since the base of the exponential is the same as the base of the logarithm:

\(\log_2 (32) = 5\)

Therefore, we can evaluate \(\log_2 (32)\) as equal to \(\boldsymbol{5}\).


Review

Move each expression into the correct category.

\(\log_{-4}15 - \log_{-4}6\)

\(2\log_4(x)\)

\(\log_{\frac{3}{4}}\left(\left(\cfrac{3}{4}\right)^{-5}\right)\)

\(\log10\)

\(\log_9(2 \cdot 8)\)

\(\log_4(4^2)\)

\(-\cfrac{1}{2}\log_{9}(x)\)

\(\log_79-\log_73\)

\(\log_{0.5}0.5\)

\(\log_38+\log_35\)

\(\log_61\)

\(\log_3\left(\cfrac{30}{6}\right)\)

\(\log_4(x^2)\)

\(\log_{-2}1\)

\(\log_{0.1}\cfrac{2}{7} + \log_{0.1}\cfrac{4}{5}\)

Zero Law

Identity Law

Inverse Property

Product Law

Quotient Law

Power Law