16.2 Laws of Logarithm

So \(a^{\displaystyle {c}}=b\iff \log _{\displaystyle {a}}b=c\)

The base of a logarithm must be positive and not equal to \(1\), and only positive numbers have logarithms — so in \(\log _{a}b\) we require \(a>0\), \(a\neq 1\) and \(b>0\).

Example 16.5.

Express the following in logarithm notation: \(4^{3}=64\). \[4^{3}=64\iff \log _{4}64=3\]

Let \(M,N, a>0\) and \(n\) be any number

1.
\(\log _{\displaystyle {a}}MN =\log _{\displaystyle {a}}M+\log _{\displaystyle {a}}N\)
2.
\(\log _{\displaystyle {a}}\frac {M}{N}=\log _{\displaystyle {a}}M-\log _{\displaystyle {a}}N\)
3.
\(\log _{\displaystyle {a}}M^{\displaystyle {n}} = n\log _{\displaystyle {a}}M\)
4.
\(\log _{\displaystyle {a}}a=1\iff a^1=a\)
5.
\(\log _{\displaystyle {a}}1=0 \iff a^{0}=1\)

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