Convexity and Concavity

This article will explore convexity and concavity, both in functions and polygons. When we talk about convexity and concavity, we are referring to the shape of the curve or function. Now, **convexity and concavity** do sound like complicated terms. However, we will learn that they are nothing so fearful- they are simply a way of describing what a curve or function looks like. So, without any further introduction, let's define what exactly concave and convex functions are.

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Jetzt kostenlos anmeldenThis article will explore convexity and concavity, both in functions and polygons. When we talk about convexity and concavity, we are referring to the shape of the curve or function. Now, **convexity and concavity** do sound like complicated terms. However, we will learn that they are nothing so fearful- they are simply a way of describing what a curve or function looks like. So, without any further introduction, let's define what exactly concave and convex functions are.

First, we will discuss concave functions.

A **concave function** is a function where a straight segment between any two points on the graph does not lie above the curve of the graph. In other words, the straight line is always below or on the curve.

Shown below is an example of a concave function. You can see that if we pick any two points on the curve, and draw a line segment between them, the line segment will always lie below the curve.

Although this example only uses two points, for a function to be concave the rule must be true for all combinations of points on that function, in the given range.

To algebraically portray that a function is concave, the following equation is used:

$\lambda x+(1-\lambda )y\u2a7d\lambda f\left(x\right)+(1-\lambda )f\left(y\right)$

In other words, suppose$x$and$y$are any two points on the x-axis upon which the function is graphed.

Before breaking this down into words, it is important to understand that$\lambda x+(1-\lambda )y$selects any point between the points$x$and$y$. Similarly, $\lambda f\left(x\right)+(1-\lambda )f\left(y\right)$ selects any point between$f\left(x\right)$and$f\left(y\right)$.

The first portion of the inequality finds the value of the function of any point between$x$and$y$. The second portion selects any point between the functions of $x$ and $y$. Thus, what this equation represents is that the function of any point between$x$and$y$is greater than or equal to any point between points$x$and$y$.

Observing the graph above, it is clear that this is true for that graph as the functions of points between the coordinates of intersection are above the functions between both functions, represented by the blue linear equation.

Remember that $\lambda $ is any and all numbers between one and zero, such that all points and functions between the original parameters are verified.

A convex function is a function where a straight segment between any two points on the graph does not lie below the curve of the graph, In other words, the straight line is always above or at the same place as the function's curve. It is the opposite of a concave function.

An example can be seen below:

It is clear that this type of function opposes a concave function. A line between two points (representing all functions between both functions) is always above or at the same level as the function itself.

Since both types of functions have similar parameters, their equations resemble each other. They have only one vital difference:

$\lambda x+(1-\lambda )y\u2a7e\lambda f\left(x\right)+(1-\lambda )f\left(y\right)$

Notice the inequality sign is flipped the other way. Since all other components are identical, this function represents that any point between the selected coordinates is greater than or equal to any point on the function between both coordinates.

Yes, this is possible. This is because both functions have an equal sign in the inequality. The most common example of this is any straight line, as the function for a point between any two points will match the equivalent function between both functions.

A concave polygon is any geometric shape where at least one internal angle exceeds 180 degrees (or $\mathrm{\pi}$radians). That is, there is a line that bends further inside than a straight line.

An example of this type of shape can be seen below:

In the above shape, the angle EDC exceeds 180 degrees. Therefore, it is a concave function.

There is a visual test that can be done to check for a concave polygon: if a straight line between any two points inside a polygon goes outside of the shape, that shape is a concave polygon. For example, below we can see that if we draw in the straight line segment $EC$, the line goes outside of the shape. Therefore, the polygon is concave.

A convex polygon is any polygon where no internal angle exceeds 180 degrees ($\mathrm{\pi}$ radians), that is, there is no internal angle that bends further than a straight line. Recall that a polygon is a shape made entirely of line segments.

An example would be the following shape:

The visual test for concave polygons can be inverted to test for a convex polygon. Since this polygon has no two points which create a segment that crosses outside of it, this geometric shape is a convex polygon.

The main difference between concavity and convexity is the fact that the angles subtended in convex shapes curve outwards whereas the angles subtended in concave shapes curve inwards. This is all based on whether or not there is an angle that exceeds 180 degrees.

Below are some further examples of concave and convex polygons. See if you can determine whether they are concave or convex.

**For the following polygons, determine whether they are concave or convex. **

**Solution:**

In the above shape, we can see that there are interior angles that exceed 180 degrees. For example, the angle KJI exceeds 180 degrees. Thus, it is concave.

For this polygon, we can see that there are also interior angles that exceed 180 degrees. For example, the angle EDC exceeds 180 degrees. Thus, it is concave.

For the above polygon, we can see that there are no interior angles that exceed 180 degrees. Thus, it is convex.

Below are some further examples of concave and convex functions. See if you can determine whether they are concave or convex.

**For the following functions, determine whether they are concave,**** convex or both.**

**Solution:**

For the above example, we have a cubic function. If we were to draw in the line segment from the point $(0,3)$ to the point $(1,6)$, it would lie above the curve. Thus, this function is convex.

Determining concavity or convexity of functions example 2- StudySmarter Originals

Now, above we have a quartic function. We can see that any line segment drawn will lie below the curve. Thus, the function is concave.

Finally, we have a straight line. Any line segment will lie on the line and hence it is both concave and convex.

- No segment created by any two points on a Concave Function will be above the function itself.
- No segment created by any two points on a Convex Function will be below the function itself.
- A function can be both Concave and Convex (e.g. a straight line).
- A Concave polygon has an internal angle greater than 180 degrees.
- A Convex polygon has no internal angle greater than 180 degrees.

Yes. An example would be a straight line.

Convex: It is when a function is above or level with a straight segment between two points that lie on the function.

Concave: It is when a function is below or level with a straight segment between two points that lie on the function.

Can a function be both concave and convex?

Yes- a straight line is both concave and convex.

Which of the following would be a convex function?

Both

Which of the following is a concave function?

Neither

Which equation is both concave and convex?

y=2x+7

A polygon with an internal angle greater than 180 degrees is:

Concave

A triangle will always be a:

convex polygon

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