Basics of Logarithmic Derivative

Basics of Logarithmic Derivative

\(\dfrac{\mathrm d}{\mathrm dx}\ln x=\dfrac1x\quad(x>0)\)

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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Answer:\(\frac{\mathrm{d}}{\mathrm{d}x}\ln x=\frac{1}{x}\), \(\frac{\mathrm{d}}{\mathrm{d}x}\ln(ax+b)=\frac{a}{ax+b}\)

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