For certain functions, it is possible to differentiate twice and find the derivative of the derivative. It is often denoted as or . For example, given that then the derivative is and the second derivative is given by .
How to classify stationary points:
The second derivative can tell us something about the nature of a stationary point. Suppose that we have found the -coordinates of all of the stationary points by solving . For a minimum, the gradient changes from negative to 0 to positive, i.e. the gradient is increasing. If the gradient is increasing then the gradient of the gradient is positive, i.e. . For a maximum, the gradient changes from positive to 0 to negative, i.e. the gradient is decreasing. If the gradient is decreasing then the gradient of the gradient is negative, i.e. . By putting the -coordinates of the stationary points into , we can classify whether they are minima or maxima by determining whether the second derivative is positive or negative at those -coordinates.
Example – Find and classify the stationary points of .
We locate the stationary points by solving . f'(x) is given by
We can solve f'(x)=0 by factorising:
which gives x=1/3 or x=1. The corresponding y coordinates are (don’t be afraid of strange fractions) and . Hence, the stationary points are at (1/3,-131/27) and (1,-5). We can classify the stationary points by substituting the x coordinate of the stationary point into the second derivative and seeing if it is positive or negative. Differentiating a second time gives
. It follows that which is less than 0, and hence (1/3,-131/27) is a MAXIMUM. and (1,-5) is a MINIMUM.