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Functions · Real functions

Parity of a Function

Before you start, make sure you know: Symmetry (High School Year 1 (semesters 1 and 2))
Before you start, make sure you know: Oriented angles and the unit circle (Secondary Year 3)

A prediction algorithm measures its error symmetrically: being off by the same amount too high or too low counts the same. How does that symmetry show up in the function, and how can you check it without graphing? That is parity, and in this section we study it in polynomials, rational functions, trigonometric functions, absolute value, exponentials, and logarithms.

Checking parity by substituting $-x$

The graph of an even function is symmetric about the $y$-axis (the left side is the mirror image of the right side). The graph of an odd function is symmetric about the origin: if you rotate the graph $180^\circ$ around the origin, it looks the same.

Comparison graphs of an even function, symmetric about the y-axis, and an odd function, symmetric about the origin. even: symmetric about the y-axis odd: symmetric about the origin
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Determine analytically whether the polynomial function $f(x) = 3x^4 - 2x^2 + 5$ is even, odd, or neither.

Determine analytically whether the polynomial function $f(x) = 3x^4 - 2x^2 + 5$ is even, odd, or neither.

Solve
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Check mathematically whether the polynomial function $h(x) = x^3 - 5x + 2$ exhibits any type of symmetry about the y-axis or about the origin.

Check mathematically whether the polynomial function $h(x) = x^3 - 5x + 2$ exhibits any type of symmetry about the y-axis or about the origin.

Solve
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Observe the following graph and classify the function shown as even, odd, or without evident symmetry, justifying your answer based on the highlighted points.

Observe the following graph and classify the function shown as even, odd, or without evident symmetry, justifying your answer based on the highlighted points.

X Y a -a f(a) f(-a)
Solve

Reading symmetry: values and graphs

If you know a function is even or odd, half the information comes for free: knowing $f(a)$ is enough to know $f(-a)$.

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Knowing that the maintenance cost of a machine follows an even function $C(t)$ with respect to the optimal time $t=0$, and that $C(4) = 3\,000$ pesos, what will be the…

Knowing that the maintenance cost of a machine follows an even function $C(t)$ with respect to the optimal time $t=0$, and that $C(4) = 3\,000$ pesos, what will be the value of $C(-4)$?

Solve
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The position of an object in a three-dimensional virtual environment is governed by an odd function $p(t)$. If at time $t=3$ s its position on one of the axes is…

The position of an object in a three-dimensional virtual environment is governed by an odd function $p(t)$. If at time $t=3$ s its position on one of the axes is $p(3) = -12$ m, what was its position at time $t=-3$ s?

Solve
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Complete the missing values so that the function $f(x)$ is odd throughout its domain: $f(1)=4$, $f(2)=-1$, $f(-1)=\dots$, $f(-2)=\dots$

Complete the missing values so that the function $f(x)$ is odd throughout its domain: $f(1)=4$, $f(2)=-1$, $f(-1)=\dots$, $f(-2)=\dots$

Solve
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The graph shows the right half (for $x \ge 0$) of the profile of a parabolic antenna. Knowing that its overall equation corresponds to an even function, describe what…

The graph shows the right half (for $x \ge 0$) of the profile of a parabolic antenna. Knowing that its overall equation corresponds to an even function, describe what the left half would look like and what value $f(-x)$ will take as a function of $f(x)$.

X Y x f(x) f(x)
Solve

Rational and trigonometric functions

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Study the parity of the rational function $g(x) = \frac{2x}{x^2 + 1}$, stating whether it exhibits symmetry about the y-axis or about the origin.

Study the parity of the rational function $g(x) = \frac{2x}{x^2 + 1}$, stating whether it exhibits symmetry about the y-axis or about the origin.

Solve
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Determine analytically the parity of the trigonometric function $f(x) = 2\sin(x)$, recalling the properties of sine for negative angles.

Determine analytically the parity of the trigonometric function $f(x) = 2\sin(x)$, recalling the properties of sine for negative angles.

Solve
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The profile of an architectural arch is modeled by the function $f(x) = \frac{x^2 - 16}{x^2 + 4}$, where $x$ is the horizontal distance from the center in meters. Prove…

The profile of an architectural arch is modeled by the function $f(x) = \frac{x^2 - 16}{x^2 + 4}$, where $x$ is the horizontal distance from the center in meters. Prove analytically whether this profile is symmetrical about the vertical central axis (even function).

Solve

Products of functions and absolute value

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Given two functions, $f(x)$ which is even and $g(x)$ which is odd, prove analytically what type of parity the product function $h(x) = f(x) \times g(x)$ has.

Given two functions, $f(x)$ which is even and $g(x)$ which is odd, prove analytically what type of parity the product function $h(x) = f(x) \times g(x)$ has.

Solve
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Study the parity of the function $f(x) = x|x|$, which describes the aerodynamic drag force of a vehicle as a function of its speed $x$.

Study the parity of the function $f(x) = x|x|$, which describes the aerodynamic drag force of a vehicle as a function of its speed $x$.

Solve
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Determine analytically whether the function $g(x) = \frac{x^3 - x}{|x|}$ is even, odd, or neither.

Determine analytically whether the function $g(x) = \frac{x^3 - x}{|x|}$ is even, odd, or neither.

Solve

Exponentials, logarithms, and roots

With other functions the method does not change: substitute $-x$ and simplify using the properties of exponents, logarithms, or roots.

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A data analytics firm uses the function $E(x) = 3^x + 3^{-x}$ to measure the symmetrical error of a prediction algorithm. Analyse and prove the parity of this function.

A data analytics firm uses the function $E(x) = 3^x + 3^{-x}$ to measure the symmetrical error of a prediction algorithm. Analyse and prove the parity of this function.

Solve
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Study the parity of the function $f(x) = \ln\left(\frac{1-x}{1+x}\right)$, used in physics to model certain electric potentials on the interval $(-1, 1)$.

Study the parity of the function $f(x) = \ln\left(\frac{1-x}{1+x}\right)$, used in physics to model certain electric potentials on the interval $(-1, 1)$.

Solve
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Analyse the parity of the function $f(x) = \sqrt{x^2 + x + 1} - \sqrt{x^2 - x + 1}$, which represents the difference in distances from a point $P(x, 0)$ on the x-axis to…

Analyse the parity of the function $f(x) = \sqrt{x^2 + x + 1} - \sqrt{x^2 - x + 1}$, which represents the difference in distances from a point $P(x, 0)$ on the x-axis to two heat emitting sources located at $F_1\left(-\frac{1}{2}, \frac{\sqrt{3}}{2}\right)$ and $F_2\left(\frac{1}{2}, \frac{\sqrt{3}}{2}\right)$.

Solve

Parameters that force a parity

If you are told that a function is even or odd, the condition $f(-x)=\pm f(x)$ must hold for every $x$. When you expand, you equate the coefficients of the powers.

Extra: vertical asymptote

Some problems with parameters give a vertical asymptote as a hint. It is a line $x=x_0$ that the graph gets closer and closer to without touching it, because the function shoots up or down near $x_0$.

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Calculate the value of the parameter $k$ so that the profit function of a company, given by $B(x) = x^2 + (k-3)x + 10$, is an even function and, therefore, symmetrical…

Calculate the value of the parameter $k$ so that the profit function of a company, given by $B(x) = x^2 + (k-3)x + 10$, is an even function and, therefore, symmetrical about $x=0$.

Solve
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The electromagnetic attraction force between two metal components is modelled using an odd function $F(d) = \frac{ad^3 + bd^2 + 5d}{d^2 + 1}$, where $d$ is the distance.…

The electromagnetic attraction force between two metal components is modelled using an odd function $F(d) = \frac{ad^3 + bd^2 + 5d}{d^2 + 1}$, where $d$ is the distance. Determine the value that the parameter $b$ must necessarily take for this symmetry to hold.

Solve
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An engineer designs a mechanical part whose upper outline is given by $f(x) = \frac{ax^3 + cx}{x^2 + b}$. Knowing that the outline is an odd function, passes through the…

An engineer designs a mechanical part whose upper outline is given by $f(x) = \frac{ax^3 + cx}{x^2 + b}$. Knowing that the outline is an odd function, passes through the points $(2, \frac{10}{3})$ and $(3, \frac{15}{4})$, and has a vertical asymptote at $x=1$ (for $x>0$), calculate the exact values of the parameters $a, b$ and $c$.

Solve
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Let $f(x)$ be a polynomial function of degree 4. If $f(x)$ is an even function, $f(0)=3$, $f(1)=4$ and $f(2)=11$, determine the full analytical expression of $f(x)$.

Let $f(x)$ be a polynomial function of degree 4. If $f(x)$ is an even function, $f(0)=3$, $f(1)=4$ and $f(2)=11$, determine the full analytical expression of $f(x)$.

Solve

Composition, decomposition, and the derivative

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Prove that any function $f(x)$ defined on $\mathbb{R}$ can be expressed as the sum of an even function $P(x)$ and an odd function $I(x)$ using the expressions…

Prove that any function $f(x)$ defined on $\mathbb{R}$ can be expressed as the sum of an even function $P(x)$ and an odd function $I(x)$ using the expressions $P(x) = \frac{f(x)+f(-x)}{2}$ and $I(x) = \frac{f(x)-f(-x)}{2}$.

Solve
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Knowing that any function $f(x)$ defined on $\mathbb{R}$ can be expressed as the sum of an even function $P(x) = \frac{f(x)+f(-x)}{2}$ and an odd function…

Knowing that any function $f(x)$ defined on $\mathbb{R}$ can be expressed as the sum of an even function $P(x) = \frac{f(x)+f(-x)}{2}$ and an odd function $I(x) = \frac{f(x)-f(-x)}{2}$, decompose the exponential function $f(x) = e^x$ into the sum of an even function and an odd function.

Solve
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If $f(x)$ is an even function and $g(x)$ is an odd function, determine and rigorously prove the parity of the composite function $h(x) = f(g(x))$.

If $f(x)$ is an even function and $g(x)$ is an odd function, determine and rigorously prove the parity of the composite function $h(x) = f(g(x))$.

Solve
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The derivative of a function gives us the gradient of its tangent line at each point. If $f(x)$ is an odd and differentiable function on all of $\mathbb{R}$, prove…

The derivative of a function gives us the gradient of its tangent line at each point. If $f(x)$ is an odd and differentiable function on all of $\mathbb{R}$, prove mathematically that its derivative function $f'(x)$ must necessarily be an even function.

Solve

Back to the beginning: if the algorithm penalizes a positive deviation the same as a negative one, its error satisfies $E(-x)=E(x)$: it is an even function, like $|x|$ or $x^2$.

Summary

See all the exercises in this section