Increasing and Decreasing Functions

Increasing and Decreasing Functions

\(\mathrm{f}^{\prime}(x)>0:\ \text{increasing}\qquad \mathrm{f}^{\prime}(x)<0:\ \text{decreasing}\)

Solve the derivative inequality on the function’s domain. Split intervals at excluded values.

In the sketch, the function is increasing for \(x<a\) or \(x>c\), and decreasing for \(a<x<c\). The derivative is zero at \(x=a\) and \(x=c\).

Changing gradient

\(\mathrm{f}^{\prime\prime}(x)>0:\ \text{gradient increasing}\qquad \mathrm{f}^{\prime\prime}(x)<0:\ \text{gradient decreasing}\)

Example

For \(\mathrm f(x)=x^3-3x^2\), \(\mathrm f'(x)=3x^2-6x=3x(x-2)\).

\(\mathrm f'(x)>0\) for \(x<0\) or \(x>2\), so \(\mathrm f\) is increasing there; \(\mathrm f'(x)<0\) for \(0<x<2\), so \(\mathrm f\) is decreasing.

Exam check: a function can be increasing for all \(x\) and still have \(\mathrm f'(x)=0\) at an isolated point, e.g. \(x^3\) at \(x=0\). Show that \(\mathrm f'(x)\ge0\) with equality only at isolated points.

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Answer:Increasing where \(\frac{\mathrm dy}{\mathrm dx}=\mathrm{f}'(x)>0\). Decreasing where \(\frac{\mathrm dy}{\mathrm dx}=\mathrm{f}'(x)<0\). In the sketch, increasing for \(x<a\) and \(x>c\), and decreasing for \(a<x<c\).

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