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Logarithmic Differentiation

Consider a function of the form y=g(x)f(x).y=g(x)^{f(x)}. To compute the derivative of this function, it is useful (and sometimes necessary) to first take the logarithm on both sides of the equation and then compute the derivative. This process is called logarithmic differentiation.

Procedure for Logarithmic Differentiation

  1. Take the natural logarithm on both sides of the equation: lny=ln (g(x)f(x))\ln y=\ln\ (g(x)^{f(x)})
  2. Simplify using log rules
  3. Differentiate Implicitly
  4. Solve
  5. Simplify
  6. Substitute given information (if asked to)
Wize Tip
You can use Logarithmic Differentiation on derivatives of complex products or quotients by using properties of logarithms.

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Example: Logarithmic Differentiation

Find the derivative of f(x)=xxf(x)=x^x.


1) lnf(x)=lnxx\text{1)} \ \ln f(x) = \ln x^x

2) lnf(x)=xlnx\text{2)} \ \ln f(x) = x \ln x
3) f(x)f(x)=1×lnx+x×1x\text{3)} \ \displaystyle \frac{f'(x)}{f(x)}=1\times\ln x + x \times\frac{1}{x}

4) f(x)=f(x)[lnx+1]\text{4)} \ f'(x)=f(x)[\ln x +1]

5) f(x)=xx[lnx+1]\text{5)} \ f'(x)=x^x[\ln x +1]

Find the derivative of f(x)=sinxcosxf(x)=\sin{x}^{\cos{x}}.

Extra Practice