For \(a,c>0,\ a,c\ne1\) and \(b>0\):
\(\log_a b=\dfrac{\log_c b}{\log_c a}\)
Examples
\(\begin{aligned} \log_9 x&=\dfrac{\log_3 x}{\log_3 9}=\dfrac{1}{2}\log_3 x\quad(x>0) \\ \log_5 18&=\dfrac{\lg18}{\lg5}=\dfrac{\ln18}{\ln5}\approx1.80\quad(3\text{ s.f.}) \end{aligned}\)
Reciprocal identity
\(\log_a b=\dfrac{1}{\log_b a}\quad(b\ne1)\)
\(\log_x3=\) \(\quad(x>0,\ x\ne1)\)
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