\(\dfrac{\mathrm d}{\mathrm dx}\ln x=\dfrac1x\quad(x>0)\)
Applying the chain rule
For \(y=\ln(ax+b)\), the inner function has derivative \(a\). On its domain \(ax+b>0\),
\(\dfrac{\mathrm{d}y}{\mathrm{d}x}\)\({}=\dfrac1{ax+b}\cdot a\)\({}=\dfrac a{ax+b}\)
Example
\(\dfrac{\mathrm d}{\mathrm dx}\ln\left(x^2+1\right)=\dfrac{2x}{x^2+1}\qquad \dfrac{\mathrm d}{\mathrm dx}\ln(3x)=\dfrac{3}{3x}=\dfrac1x\)
Exam check: use the log laws first where they help, e.g. \(\ln(3x)=\ln3+\ln x\) and \(\ln3\) is a constant, so its derivative is \(0\).
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