Concavity of a Function
What is concavity?
- Concavity is the way in which a curve (or surface) bends
- Mathematically,
- a curve is CONCAVE DOWN if for all values of in an interval
- a curve is CONCAVE UP if for all values of in an interval
- CONCAVE DOWN is often called concave
- CONCAVE UP is often called convex
Exam Tip
- In an exam an easy way to remember the difference is:
- Concave down is the shape of (the mouth of) a sad smiley ☹︎
- Concave up is the shape of (the mouth of) a happy smiley ☺︎
Worked Example
The function is given by.
a)
Determine whether the curve of the graph of is concave down or concave up at the points where and.
b)
Find the values of for which the curve of the graph of is concave up.
Points of Inflection
What is a point of inflection?
- A point at which the curve of the graph of changes concavity is a point of inflection
- The alternative spelling, inflexion, may sometimes be used
What are the conditions for a point of inflection?
- A point of inflection requires BOTH of the following two conditions to hold
-
- the second derivative is zero
- the second derivative is zero
AND
-
- the graph of changes concavity
- changes sign through a point of inflection
- the graph of changes concavity
- It is important to understand that the first condition is not sufficient on its own to locate a point of inflection
- points where could be local minimum or maximum points
- the first derivative test would be needed
- However, if it is already known has a point of inflection at, say, then
- points where could be local minimum or maximum points
What about the first derivative, like with turning points?
- A point of inflection, unlike a turning point, does not necessarily have to have a first derivative value of 0 ( )
- If it does, it is also a stationary point and is often called a horizontal point of inflection
- the tangent to the curve at this point would be horizontal
- The normal distribution is an example of a commonly used function that has a graph with two non-stationary points of inflection
- If it does, it is also a stationary point and is often called a horizontal point of inflection
How do I find the coordinates of a point of inflection?
- For the function
STEP 1
Differentiate twice to find and solve to find the -coordinates of possible points of inflection
STEP 2
Use the second derivative to test the concavity of either side of
- If then is concave down
- If then is concave up
If concavity changes, is a point of inflection
STEP 3
If required, the-coordinate of a point of inflection can be found by substituting the-coordinate into
Exam Tip
- You can find the x-coordinates of the point of inflections of by drawing the graph and finding the x-coordinates of any local maximum or local minimum points
- Another way is to draw the graph and find the x-coordinates of the points where the graph crosses (not just touches) the x-axis
Worked Example
Find the coordinates of the point of inflection on the graph of.
Fully justify that your answer is a point of inflection.