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Math II · Mathematics
Functions · Functions

Even and Odd Functions (Symmetry)

Before you start, make sure you know: Characteristics of a function (Integrated Math I)
Before you start, make sure you know: Operations with powers (Math 8 (Pre-Algebra))

Picture the main cable of the Golden Gate Bridge. It dips to the same height on both sides of the center tower line. So if you measure the cable 500 feet to the left of center, do you also know its height 500 feet to the right? Symmetry lets you answer that without a second measurement, and it cuts the work of graphing in half. In this lesson you will learn to prove symmetry with algebra instead of trusting your eyes.

Even functions: mirror image across the y-axis

Fold the graph along the $y$-axis. If the left half lands exactly on the right half, the function is even.

Graph of an even function symmetric about the y-axis, showing the mirror points (−a, f(a)) and (a, f(a)) at the same height. (−a, f(a))(a, f(a)) axis of symmetry: x = 0

To test algebraically, replace every $x$ by $(-x)$, simplify, and compare with the original. The key fact is how powers behave:

Absolute value works the same way: for $f(x) = |x| - 3$ you get $f(-x) = |-x| - 3 = |x| - 3 = f(x)$, so it is even.

Slide $b$ in the graph above. The parabola is symmetric about the $y$-axis only when the $x$-term disappears ($b = 0$).

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Look at the graphs of the functions shown in panels A and B. State what type of symmetry (even, odd, or neither) each of them has, justifying your answer based on their…

Look at the graphs of the functions shown in panels A and B. State what type of symmetry (even, odd, or neither) each of them has, justifying your answer based on their visual shape.

X Y A X Y B
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Given the polynomial function $f(x) = 3x^2 - 5$, calculate the value of $f(-x)$. From the result, state whether the function has even, odd, or no symmetry.

Given the polynomial function $f(x) = 3x^2 - 5$, calculate the value of $f(-x)$. From the result, state whether the function has even, odd, or no symmetry.

Solve
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Consider the absolute value function $f(x) = |x| - 3$. Calculate the expression for $f(-x)$ and determine whether the function has any type of symmetry.

Consider the absolute value function $f(x) = |x| - 3$. Calculate the expression for $f(-x)$ and determine whether the function has any type of symmetry.

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Odd functions: symmetric about the origin

Now imagine spinning the graph half a turn (180°) around the origin. If it lands on itself, the function is odd.

Graph of an odd function symmetric about the origin, showing the opposite points (a, f(a)) and (−a, −f(a)). (−a, −f(a))(a, f(a))

For a polynomial, "odd function" means every term has an odd exponent and there is no constant term.

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Given the function $g(x) = -2x^3$, calculate the expression for $g(-x)$. What type of symmetry does this function have?

Given the function $g(x) = -2x^3$, calculate the expression for $g(-x)$. What type of symmetry does this function have?

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Analyze the linear function $f(x) = 4x$. Calculate $f(-x)$ and determine algebraically whether it is an even function, an odd function, or if it has no symmetry.

Analyze the linear function $f(x) = 4x$. Calculate $f(-x)$ and determine algebraically whether it is an even function, an odd function, or if it has no symmetry.

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Given the polynomial function $f(x) = x^3 - 4x$, calculate the numerical values of $f(2)$ and $f(-2)$. What do these results suggest about the symmetry of the function?…

Given the polynomial function $f(x) = x^3 - 4x$, calculate the numerical values of $f(2)$ and $f(-2)$. What do these results suggest about the symmetry of the function? Prove your hypothesis algebraically for any value of $x$.

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Neither: no symmetry, and the domain check

Most functions are neither even nor odd. Symmetry is the exception.

There is one more requirement. For the equation $f(-x) = f(x)$ to make sense, both $x$ and $-x$ must be in the domain.

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Consider the linear function $f(x) = 2x + 1$. Check algebraically whether it satisfies the condition for even symmetry ($f(-x) = f(x)$) or odd symmetry ($f(-x) = -f(x)$).

Consider the linear function $f(x) = 2x + 1$. Check algebraically whether it satisfies the condition for even symmetry ($f(-x) = f(x)$) or odd symmetry ($f(-x) = -f(x)$).

Solve
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A student claims that the function $f(x) = \frac{1}{x-2}$ is odd because the $x$ in the denominator is raised to an odd exponent (1). Is this claim correct? Prove or…

A student claims that the function $f(x) = \frac{1}{x-2}$ is odd because the $x$ in the denominator is raised to an odd exponent (1). Is this claim correct? Prove or refute it using algebraic analysis.

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Rigorously analyze the symmetry of the rational function $f(x) = \frac{x^3 - x}{x^2 - 4}$. Show all the algebraic steps.

Rigorously analyze the symmetry of the rational function $f(x) = \frac{x^3 - x}{x^2 - 4}$. Show all the algebraic steps.

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Using symmetry to find values and points

Symmetry gives you free information. Knowing one point on the graph gives you a second point.

A point (a, b) with its image (−a, b) for an even function and its image (−a, −b) for an odd function. (a, b) even: (−a, b) odd: (−a, −b)
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It is known that a function $h(x)$ has even symmetry. If the graph of this function passes through the point $(4, 7)$, what other point must necessarily belong to its…

It is known that a function $h(x)$ has even symmetry. If the graph of this function passes through the point $(4, 7)$, what other point must necessarily belong to its graph?

Solve
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A function $p(x)$ has odd symmetry. If we know that $p(2) = -5$, what must the value of $p(-2)$ be? Write the coordinates of the corresponding point.

A function $p(x)$ has odd symmetry. If we know that $p(2) = -5$, what must the value of $p(-2)$ be? Write the coordinates of the corresponding point.

Solve
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A company logo has been designed using a mathematical function that is symmetrical about the origin. If the outline of the logo passes through the point $(-3, 6)$, what…

A company logo has been designed using a mathematical function that is symmetrical about the origin. If the outline of the logo passes through the point $(-3, 6)$, what other point will it definitely pass through?

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Symmetry in graphs and real-world models

In a model, symmetry has a meaning you can describe in words:

  • Even: the input's direction (sign) doesn't matter. The output is the same either way.

  • Odd: reversing the input's direction reverses the output.

When a graph is given and the function is known to be even, you can read the left side from the right side. For $x = -3$, find the height at $x = 3$ and use the same value.

Back to the start: the Golden Gate cable is modeled by an even function, because its shape is a mirror image about the center line. That's why the height 500 feet to the left equals the height 500 feet to the right.

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The path of a pendulum as a function of time $t$ is modeled by a function $p(t)$. Knowing that $p(t)$ is an odd function and that at $1.5$ seconds its position is…

The path of a pendulum as a function of time $t$ is modeled by a function $p(t)$. Knowing that $p(t)$ is an odd function and that at $1.5$ seconds its position is $p(1.5) = 4$ inches, what will its position be at $t = -1.5$ seconds? Explain the physical meaning of this result.

Solve
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The cable of a suspension bridge takes the shape of a parabola described by the equation $y = 0.05x^2 + 10$. Verify algebraically whether this function has even, odd, or…

The cable of a suspension bridge takes the shape of a parabola described by the equation $y = 0.05x^2 + 10$. Verify algebraically whether this function has even, odd, or no symmetry, and interpret the result geometrically.

Solve
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The following graph shows the temperature variation $T$ in a chemical experiment as a function of time $t$, recorded from the instant $t = 0$. If the mathematical model…

The following graph shows the temperature variation $T$ in a chemical experiment as a function of time $t$, recorded from the instant $t = 0$. If the mathematical model $T(t)$ governing this experiment is an even function over its entire real domain, what will the temperature be at the instant $t = -3$? Justify your answer.

t T 3 11 T(t)
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Rational functions

For a rational function, substitute $-x$ in the numerator and in the denominator separately. Watch how each odd power changes sign. And remember to check the domain first.

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Check algebraically the type of symmetry that the rational function $f(x) = \frac{5x}{x^2 + 2}$ has.

Check algebraically the type of symmetry that the rational function $f(x) = \frac{5x}{x^2 + 2}$ has.

Solve
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Determine whether the function $g(x) = \frac{x^2 - 4}{x^2 + 1}$ is even, odd, or neither. Show all the steps of your reasoning.

Determine whether the function $g(x) = \frac{x^2 - 4}{x^2 + 1}$ is even, odd, or neither. Show all the steps of your reasoning.

Solve
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The power generated by a wind turbine is modeled by the function $P(v) = \frac{v^3}{v^2 + 10}$, where $v$ represents the wind speed. Study analytically the symmetry of…

The power generated by a wind turbine is modeled by the function $P(v) = \frac{v^3}{v^2 + 10}$, where $v$ represents the wind speed. Study analytically the symmetry of $P(v)$. If we consider negative wind speeds as wind blowing in the opposite direction, what physical meaning does the obtained symmetry have with respect to the generated power?

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Special cases and combining functions

Can a function be both even and odd? Write both definitions together:

You can also build new functions from known ones.

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Knowing that $f(x)$ is an odd function and that it is defined at the point $x = 0$, what must the value of $f(0)$ necessarily be? Prove this using the definition of an…

Knowing that $f(x)$ is an odd function and that it is defined at the point $x = 0$, what must the value of $f(0)$ necessarily be? Prove this using the definition of an odd function.

Solve
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To predict a financial trend, a model is created by multiplying two simpler functions: $h(x) = f(x) \cdot g(x)$. Knowing that $f(x) = x^2$ (an even function) and…

To predict a financial trend, a model is created by multiplying two simpler functions: $h(x) = f(x) \cdot g(x)$. Knowing that $f(x) = x^2$ (an even function) and $g(x) = x$ (an odd function), determine algebraically what type of symmetry the resulting function $h(x)$ will have.

Solve
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Suppose there is a function $f(x)$ that is simultaneously even and odd over its entire domain. Set up the system of equations derived from the definitions of both…

Suppose there is a function $f(x)$ that is simultaneously even and odd over its entire domain. Set up the system of equations derived from the definitions of both symmetries and solve it to find the only function that satisfies this condition.

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Finding unknown parameters

Sometimes a function has an unknown constant, and you must choose it so the function has a certain symmetry. The plan:

  1. Compute $f(-x)$.

  2. Set $f(-x) = f(x)$ (even) or $f(-x) = -f(x)$ (odd).

  3. Compare terms with the same power of $x$ and solve for the parameter.

In the graph above, the parabola is symmetric about the $y$-axis only for one value of $k$. Which one? The $x$-coefficient $k - 3$ must be $0$, so $k = 3$.

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Find the value that the parameter $k$ must take for the function $f(x) = x^2 + kx + 5$ to be an even function. Justify your answer algebraically.

Find the value that the parameter $k$ must take for the function $f(x) = x^2 + kx + 5$ to be an even function. Justify your answer algebraically.

Solve
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Determine the value of the parameter $m$ for the polynomial function $f(x) = 2x^3 + m x^2 - x$ to have odd symmetry.

Determine the value of the parameter $m$ for the polynomial function $f(x) = 2x^3 + m x^2 - x$ to have odd symmetry.

Solve
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Calculate the value of the parameter $b$ for the rational function $f(x) = \frac{5x^2 + bx - 2}{x}$ to be an odd function.

Calculate the value of the parameter $b$ for the rational function $f(x) = \frac{5x^2 + bx - 2}{x}$ to be an odd function.

Solve

Summary

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