Laws of Logarithms
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Theorem
Let $x, y, b \in \R_{>0}$ be (strictly) positive real numbers.
Let $a \in \R$ be any real number such that $a > 0$ and $a \ne 1$.
Let $\log_a$ denote the logarithm to base $a$.
Then:
Change of Base of Logarithm
- $\log_b x = \dfrac {\log_a x} {\log_a b}$
Sum of Logarithms
Let $x, y, b \in \R$ be strictly positive real numbers such that $b > 1$.
Then:
- $\log_b x + \log_b y = \map {\log_b} {x y}$
where $\log_b$ denotes the logarithm to base $b$.
Logarithm of Power/Natural Logarithm
Let $x \in \R$ be a strictly positive real number.
Let $a \in \R$ be a real number such that $a > 1$.
Let $r \in \R$ be any real number.
Let $\ln x$ be the natural logarithm of $x$.
Then:
- $\map \ln {x^r} = r \ln x$
Logarithm of Power/General Logarithm
Let $x \in \R$ be a strictly positive real number.
Let $a \in \R$ be a real number such that $a > 1$.
Let $r \in \R$ be any real number.
Let $\log_a x$ be the logarithm to the base $a$ of $x$.
Then:
- $\map {\log_a} {x^r} = r \log_a x$
Difference of Logarithms
- $\log_b x - \log_b y = \map {\log_b} {\dfrac x y}$
Logarithm of Reciprocal
- $\map {\log_b} {\dfrac 1 x} = -\log_b x$
Examples
Arbitrary Example
- $\ln \dfrac {A^p + B^q + C^r} {D^s E^t} = p \ln A + q \ln B + r \ln C - s \ln D - t \ln E$