How to Know If a Function is Symmetric?

To determine if a function is symmetric, we have to look at its graph and identify some characteristics that are unique to symmetric functions. For example, the graph can have a reflection on the x-axis, on the y-axis, or it can have rotational symmetry about the origin.

In this article, we will look at the different types of symmetry with examples to illustrate the ideas.

ALGEBRA
how to know if a function is symmetric

Relevant for

Learning to determine if a function has some kind of symmetry.

See graphs

ALGEBRA
how to know if a function is symmetric

Relevant for

Learning to determine if a function has some kind of symmetry.

See graphs

How to determine if a graph is symmetric?

We can determine if a graph is symmetric with respect to a line or a point if the graph does not change when it is reflected with respect to that line or is rotated around that point.

Symmetry can be useful when we want to graph an equation as it tells us that if we know a portion of the graph, then we will also know the remaining symmetric portion of the graph.

We can distinguish three main types of symmetry:

1. A graph has symmetry about the x-axis if when we have the point (a, b) on the graph, we also have the point (a, -b). The following is a graph with symmetry about the x-axis:

graph symmetric about the x axis

2. A graph has symmetry about the y-axis if when we have the point (a, b) on the graph, we also have the point (-a, b). The following is a graph with symmetry about the y-axis:

graph symmetric about the y axis

3. A graph has symmetry about the origin if when we have the point (a, b) on the graph, we also have the point (-a, -b). The following is a graph that has symmetry with respect to the origin:

graph symmetric about the origin

Keep in mind that most graphs do not have any kind of symmetry. Furthermore, it is also possible for a graph to have more than one type of symmetry. For example, the graph of a circle centered at the origin has all three types of symmetries at the same time:

graph with all symmetries

How to know if a function has odd or even symmetry?

We can distinguish two types of symmetries in the graphs of functions:

Reflection symmetry about the y-axis. This happens when we have $latex f(x)=f(-x)$.

Rotational symmetry about the origin. This happens when we have $latex f(-x)=-f(x)$.

We observe that functions like $latex f(x) ={{x}^2}$ and $latex f(x)={{x}^4}$, where the exponent of x is even, will have the property that $latex f(x)=f(-x)$ since -1 raised to an even exponent equals 1.

Similarly, functions like $latex f(x)=x$ and $latex f(x)={{x}^3}$, where the exponent in x is odd, will have the property that $latex f(-x)=-f(x)$ given that -1 raised to an odd exponent is equal to -1.

Therefore, we have the following definitions:

A function is even if $latex f(-x)=f(x)$. An even function has a reflection about the y-axis.

graph of even function 1

A function is odd if $latex f(-x)=-f(x)$. An odd function has rotational symmetry about the origin.

graph of odd function 1

We can algebraically determine whether a function is even, odd, or none by replacing x with -x and calculating f (-x). If we have $latex f(-x)=f(x)$, the function is even. If we have $latex f(-x)=-f(x)$, the function is odd.


How to know if a function is symmetric about the origin?

Symmetry with respect to the origin is perhaps the most difficult to identify. To know if a function is symmetric with respect to the origin, we can identify several points on the graph since, in a function graph with symmetry with respect to the origin, we have the point (a, b) and the point (-a, -b).

For example, in the following graph, we have the points (2, 4) and (-2, -4). This means that the graph is symmetric with respect to the origin.

how to know if a function is symmetric 2

Test for symmetry about the origin

A graph will have symmetry with respect to the origin if we obtain an equivalent equation when all y‘s are replaced by -y and all x‘s are replaced by –x.

EXAMPLES

  • Determine whether the function $latex y=2{{x}^3}-{{x}^5}$ is symmetric about the origin.

Solution: We have to replace x with -x and y with -y.

$latex y=2{{x}^3}-{{x}^5}$

$latex -y=2{{(-x)}^3}-{{(-x)}^5}$

$latex -5=-2{{x}^3}+{{x}^5}$

We can see that all the signs in this expression are exactly opposite to those in the original expression. This means that this expression is equivalent to the original expression since we can simply multiply the entire expression by -1 to get the original expression.

  • Is the function $latex y={{x}^2}-6{{x}^4}+2$  symmetric about the origin?

Solution: Again, we have to replace x with -x and y with -y.

$latex y={{x}^2}-6{{x}^4}+2$

$latex -y={{(-x)}^2}-6{{(-x)}^4}+2$

$latex -y={{x}^2}-6{{x}^4}+2$

We remember that a negative number raised to an even power is equal to a positive number. We see that in this case, the right side of the expression is identical to the original equation, but the left side is different.

Therefore, the expression is not the same and the function has no symmetry with respect to the origin.


Examples of graphs with symmetry

In the description of the following graphs, it is shown whether there is any symmetry. Moreover, it is indicated if the graph represents a function.

The following graph is symmetric about its axis, that is, it is symmetric about the line $latex x=2$. There is no other symmetry. The graph does represent a function.

graph with symmetry about x equals 2

The graph below is symmetric about both the x-axis and the y-axis. This graph is also symmetric about the origin. Since the graph does not pass the vertical line test, that is, a vertical line can be drawn that passes through more than one point on the graph, then the graph does not represent a function.

graph symmetric about the x and y axis

This graph is symmetric about the lines x=1 and y=1. Since a vertical line can be drawn that passes through more than one point on the graph, the graph does not represent a function.

graph symmetric about x is 1 and y is 1

The graph below represents a cubic function that is symmetric about the point (1, -2). This graph does represent a function.

graph of a function symmetric about a point

In the description of the following graphs, it is shown if the function is even, odd, or none.

The following parabola has the vertex on the y-axis. This means that the axis of symmetry is the y-axis. Therefore, the function is even.

graph of a negative parabola symmtric

The following cubic function is centered at the origin and if we rotate it 180°, we will obtain the same graph. This means that the function is odd.

graph of odd function with symmetry about the origin

The following cubic function is centered at the point (0, -2). This function is symmetric, but not about the origin or the y-axis. Therefore, this function is neither even nor odd.

graph of function that is neither odd nor even

See also

Interested in learning more about functions? Take a look at these pages:

Learn mathematics with our additional resources in different topics

LEARN MORE