What is a stationary point?
Stationary points are where the gradient of a curve is equal to zero. They are divided up into three sub-groups: Maximums, Minimums, and inflection points.
At a maximum point, the value of Y is always greater than the values of Y immediately before and after its position on the X-axis.
At a minimum point, the value of Y is always less than the values of Y immediately before and after its position on the X-axis.
At an inflection point, the value of Y is either greater than the value of Y immediately before it and less than the value of Y immediately after it, or the value of Y is less than the value immediately before it and greater than the Y value immediately after it.
Finding stationary points
First of all, you must find out if there are any stationary points in the line. To do this you differentiate the function.
For this example, we’ll use the following function:
Here it is shown on a graph:
As you can see, there are two stationary points (a maximum and a minimum). Normally, there wouldn’t be a visual aid, but for the sake of this example, corresponding graphs have been included.
Using your eye, you can roughly tell that the stationary points are at -1, and 3/2 on the x-axis.
Now let’s go back to the algebraic function, and differentiate it.
Here’s the new term shown also shown as a graph:
Now lets compare both of the functions together:
As you can see, the points at which the derived function have a Y value of 0, are also where the stationary points are on the original function. This is why the first step of finding stationary points, is finding the derivative of the function.
Now, back to the derived equation.
We must now set Y to equal zero, and then solve the quadratic.
Therefore at the stationary points, x equals:
Now we take these values, and substitute them back into the original function to find the Y coordinates:
Now we know that the stationary points on the curve are at the following coordinates:
Finding out what type of stationary point it is
Now that we know how to find the stationary points, we need to know what type of stationary point it is, without the aid of a graph.
To do this we first find the second derivative of the original function. This is represented by the following:
All this is, is the derivative of the original function, differentiated again. Here’s the example we used:
Now all we do is substitute the x values we found earlier into the formula.
If the resulting number is:
> 0 it is a minimum.
< 0 it is a maximum.
= 0 it is an inflection point.
Therefore at ( -1 , 9 ), there is a maximum, and at ( 2/3 , -7/27 ), there is a minimum.