Logarithmic Functions | Fundamental Integration Formulas

The limitation of the Power Formula $\displaystyle \int u^n \, du = \dfrac{u^{n + 1}}{n + 1} + C$, is when $n = -1$; this makes the right side of the equation indeterminate. This is where the logarithmic function comes in, note that $\displaystyle \int u^{-1} \, du = \displaystyle \int \frac{du}{u}$, and we can recall that $d(\ln u) = \dfrac{du}{u}$. Thus,

$\displaystyle \int \dfrac{du}{u} = \ln u + C$


The formula above involves a numerator which is the derivative of the denominator. The denominator $u$ represents any function involving any independent variable. The formula is meaningless when $u$ is negative, since the logarithms of negative numbers have not been defined. If we write $u = -v$ so that $du = -dv$, then we have

$\displaystyle \int \dfrac{du}{u} = \int \dfrac{-dv}{-v} = \int \dfrac{dv}{v} = \ln v + C = \ln (-u) + C$

When negative numbers are involved, the formula should be considered in the form

$\displaystyle \int \dfrac{du}{u} = \ln | u | + C$

The integral of any quotient whose numerator is the differential of the denominator is the logarithm of the denominator.

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