Theorem: log_a{x} = \dfrac{log_b{x}}{log_b{a}}
Prerequisites:
Logarithm (definition)
Log of powers property (proof)
Proof:
Let a = b^z. Then by definition of log,
\qquad\quad log_b{a} = z\qquad\qquad\qquad\qquad\qquad\qquad\cdots (1)
Let log_a{x} = y, then again by definition of log:
\qquad\quad\begin{align} x &= a^y\\
&= \big(b^{y}\big)^z\\
&= b^{yz}\qquad\qquad\qquad\qquad\qquad\qquad\quad\text{(by log of powers property)}\end{align}
Now, taking log on both sides:
\qquad\quad\begin{align} log_b{x} &= yz\\
&= log_a{x}\;log_b{a}\qquad\qquad\qquad\qquad\text{(from (1))}\end{align}
\Rightarrow\quad\;\; log_a{x} = \dfrac{log_b{x}}{log_b{a}}
Hence the result.
Recommended:
Log of product
Log of powers
Multiplication of log
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