Math Doubts

Logarithmic Product identity


$\log_{b}{(m \times n)}$ $\,=\,$ $\log_{b}{m}+\log_{b}{n}$


The logarithm of product of quantities equals to sum of their logs is called the product rule of logarithms.

Math form

Let $m$ and $n$ denote two literal quantities, and $b$ represents another quantity in algebraic form. The product of the quantities $m$ and $n$ is written as either $m \times n$ or $m.n$ in mathematics, and the log of their product to the base $b$ is written in the following mathematical forms.

$(1).\,\,$ $\log_{b}{(m \times n)}$

$(2).\,\,$ $\log_{b}{(m.n)}$

The logarithms of both literal quantities $m$ and $n$ to the base $b$ are written mathematically as follows.

$(1).\,\,$ $\log_{b}{(m)}$

$(2).\,\,$ $\log_{b}{(n)}$

According to the logarithms, the logarithm of a product is equal to the sum of their logarithms. Therefore, the mathematical relation between them is expressed in mathematics as follows.

$\log_{b}{(m.n)}$ $\,=\,$ $\log_{b}{(m)}+\log_{b}{(n)}$

This mathematical property is called the product identity of the logarithms.


$\log_{b}{(m \times n \times o \times p \cdots)}$ $\,=\,$ $\log_{b}{m}$ $+$ $\log_{b}{n}$ $+$ $\log_{b}{o}$ $+$ $\log_{b}{p}$ $+$ $\cdots$


Let’s learn how to verify the product law of the logarithms by an arithmetic example.

Take $m = 2$ and $n = 3$

$\log_{\displaystyle 10} 2 = 0.3010$ and $\log_{\displaystyle 10} 3 = 0.4771$. Now add both of them.

$\log_{\displaystyle 10} 2 + \log_{\displaystyle 10} 3 = 0.3010 + 0.4771$

$\log_{\displaystyle 10} 2 + \log_{\displaystyle 10} 3 = 0.7781$

Now calculate logarithm of product of both numbers

$\log_{\displaystyle 10} (2 \times 3) = \log_{\displaystyle 10} (6)$

$\implies \log_{\displaystyle 10} 6 = 0.7781$

$\therefore \,\,\,\,\, \log_{\displaystyle 10} (2 \times 3)$ $=$ $\log_{\displaystyle 10} 2 + \log_{\displaystyle 10} 3$ $=$ $0.7781$


Learn how to derive the logarithm of product of two or more quantities is equal to the sum of their logs.


Learn how to use property of product law of logarithms in mathematics.

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