Summary and examples
is the derivative of a function in a point and is the slope of the tangent line at that point of the graph of that function.
When it means that the tangent line in has a slope and thus that it is horizontal to the -axis. For such a point there are three possibilities:
- the point is a (local) maximum;
- the point is a (local) minimum;
- the point is a horizontal inflection point (an example is the graph of ).
When we want to know in which points a function has a local maximum, minimum or horizontal inflection point, we calculate the derivative and try to find for which values , for these values of the derivative equals zero and the tangent line is horizontal.
Example 1
Determine the minimum of the parabola:
We already know that this parabola 'opens up' (the coefficient of the square equals and thus ) and thus has a minimum that lies on the symmetry-axis, see Quadratic functions and graphs.
We differentiate the function and get:
and thus:
The parabola has a minimum in the point .
Example 2
Quadratic functions (the graphs are parabolas):
have a maximum or minimum that lies on the symmetry-axis:
We can also calculate this by differentiation. The derivative of the function above is:
The derivative is equal to when:
which is the -coordinate of the top.
The values of for which the derivative of the function equals are candidates for a maximum, minimum or horizontal inflection point, but we do not know yet which of the three is the case. This requires further investigation. There are two different methods for this.
Method 1: consider the sign of the first derivative
The derivative of a maximum in a point is equal to .
Left to the maximum the function is increasing and thus for .
Right to the maximum the function is decreasing and thus for .
So the sign of the first derivative around the maximum has the following signs:
maximum : + -.
Similarly, the sign of the first derivative around the minimum has the following signs:
minimum: - +.
For a horizontal inflection point we find similarly:
horizontal inflection point : + + or - -.
Method 2: consider the 2nd derivative
When is one of the solutions of the equation it is a candidate for a maximum, minimum or horizontal inflection point. Then the following holds.
If:
, then is a minimum;
, then is a maximum;
, then is a horizontal inflection point.
Example 3
Check whether the function:
has a maximum and/or a minimum.
The derivative is:
The derivative equals if:
so if:
or
The corresponding points on the graph are, respectively:
or
Now we check both methods.
Method 1
Around we find the following signs of :
+ - and thus there is a maximum.
Around we find the following signs of :
- + and thus there is a minimum.
Method 2
We have to calculate the second derivative:
Then:
and thus the function has a maximum for
and
and thus the function has a minimum for
Example 4
Check whether the function:
has a local maximum or minimum.
The derivative of the function is, using the product rule:
The derivative equals when and thus the point is a candidate for either a maximum or minimum. Further research should reveal which of the two it is. It is even possible that it is a point of inflection.
We can use either method, but the second method is more laborious because then we need to use the production rule again. Therefore we take method 1 and only need to consider the sign of because the exponential function is always positive.
If then is negative and if then is positief and thus we have a minimum, see the figure.
Example 5
Investigate the function:
We calculate the derivative:
The equation:
has as a solution which is a candidate for a maximum, minimum of horizontal inflection point.
The second derivative is:
which is equal for . So the point is a horizontal inflection point.