Inverse Functions
A weather app says tomorrow's high in Phoenix is 86 °F, but your science teacher wants the temperature in °C. You know the formula $F = \tfrac{9}{5}C + 32$ turns Celsius into Fahrenheit, but you need to go the other way. Is there a formula that does exactly the opposite? There is: it is the inverse function, and by the end of this page you will be able to build one for almost any function.
Undoing a function
A function is a machine: you put in an input and get one output. An inverse function is the machine that runs backward. It takes the output and gives back the original input.
Not every function can be undone. If two different inputs give the same output, the reverse machine would not know which input to return.
To check that two functions really are inverses, compose them in both orders.
Prove, using the composition of functions, whether $f(x) = \frac{x+1}{2}$ and $g(x) = 2x - 1$ are inverse functions. You must verify that $(f \circ g)(x) = x$ and that…
Prove, using the composition of functions, whether $f(x) = \frac{x+1}{2}$ and $g(x) = 2x - 1$ are inverse functions. You must verify that $(f \circ g)(x) = x$ and that $(g \circ f)(x) = x$.
Finding the inverse algebraically
Think of $f(x) = 2x + 3$ as two steps: double, then add 3. To undo it, reverse the steps in reverse order: subtract 3, then divide by 2. Solving for $x$ does exactly that.
When the formula has a radical, the domain of the inverse matters.
Find the analytical expression of the inverse function of the linear function $f(x) = 3x - 5$.
Find the analytical expression of the inverse function of the linear function $f(x) = 3x - 5$.
Determine the inverse function of the rational function $g(x) = \frac{4}{x+2}$.
Determine the inverse function of the rational function $g(x) = \frac{4}{x+2}$.
Calculate the inverse function of $h(x) = \sqrt{2x - 1}$, indicating the domain of the new function obtained.
Calculate the inverse function of $h(x) = \sqrt{2x - 1}$, indicating the domain of the new function obtained.
The graph of $f^{-1}$: symmetry, domain, and range
Solving for $x$ swaps the roles of input and output. So every point $(a, b)$ on the graph of $f$ becomes the point $(b, a)$ on the graph of $f^{-1}$.
Here $f(x) = \sqrt{x}$ has domain $[0, \infty)$ and range $[0, \infty)$. Its inverse is $f^{-1}(x) = x^2$ for $x \ge 0$. The point $(4, 2)$ on $f$ matches $(2, 4)$ on $f^{-1}$.
Domain and range are not always equal, so the swap is easy to see in a second example:
| Function | Domain | Range |
|---|---|---|
| $f(x) = \sqrt{x - 1}$ | $[1, \infty)$ | $[0, \infty)$ |
| $f^{-1}(x) = x^2 + 1$ | $[0, \infty)$ | $[1, \infty)$ |
Given the graph of the exponential function $f(x) = 2^x$, draw the coordinate axes and the line $y = x$ in your notebook, and sketch the graph of its inverse function…
Given the graph of the exponential function $f(x) = 2^x$, draw the coordinate axes and the line $y = x$ in your notebook, and sketch the graph of its inverse function $f^{-1}(x)$. Identify the coordinates of at least three points through which $f^{-1}(x)$ passes.
Exponential and logarithmic functions
You already know that $2^4 = 16$. The same fact written with a logarithm is $\log_2 16 = 4$. In general, a logarithm is an exponent, so $b^x$ and $\log_b x$ undo each other.
The exponential has domain $\mathbb{R}$ and range $(0, \infty)$. By the swap rule, the logarithm has domain $(0, \infty)$ and range $\mathbb{R}$.
Obtain the inverse function of the exponential function $f(x) = 2^{x-3}$.
Obtain the inverse function of the exponential function $f(x) = 2^{x-3}$.
Find the inverse function of the logarithmic function $g(x) = \log_3(x) + 1$.
Find the inverse function of the logarithmic function $g(x) = \log_3(x) + 1$.
The sound intensity level in decibels (dB) perceived by the human ear is calculated using the function $\beta(I) = 10 \log_{10} \left( \frac{I}{I_0} \right)$, where $I$…
The sound intensity level in decibels (dB) perceived by the human ear is calculated using the function $\beta(I) = 10 \log_{10} \left( \frac{I}{I_0} \right)$, where $I$ is the acoustic intensity in $\text{W/m}^2$ and $I_0 = 10^{-12} \text{ W/m}^2$ is the threshold of hearing intensity. Find the inverse function $I(\beta)$ that determines the intensity as a function of the decibels. If the human threshold of pain is situated at $120 \text{ dB}$, what is the corresponding acoustic intensity?
Rational functions: $x$ appears twice
For $f(x) = \frac{4}{x+2}$ the letter $x$ appears once, so solving is easy. In $f(x) = \frac{3x+1}{x-2}$, $x$ appears in both numerator and denominator. The strategy is:
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Multiply both sides by the denominator.
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Distribute.
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Get all the terms that contain $x$ on one side.
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Factor out $x$ and divide.
Look at those excluded values. The function $f$ has a vertical asymptote at $x = 2$ and a horizontal asymptote at $y = 3$ (ratio of leading coefficients, $\frac{3}{1}$). The inverse $f^{-1}$ has them swapped: a vertical asymptote at $x = 3$ and a horizontal asymptote at $y = 2$. So the range of $f$ is $y \ne 3$, which is exactly the domain of $f^{-1}$.
Given the rational function $f(x) = \frac{2x - 1}{x + 3}$, find its inverse function $f^{-1}(x)$. Next, calculate the domain of $f(x)$ and the domain of $f^{-1}(x)$, and…
Given the rational function $f(x) = \frac{2x - 1}{x + 3}$, find its inverse function $f^{-1}(x)$. Next, calculate the domain of $f(x)$ and the domain of $f^{-1}(x)$, and explain the relationship that exists between these domains and the ranges of both functions.
The monthly production cost in dollars of $x$ units of an electronic component is given by the function $C(x) = \frac{120x + 500}{x + 2}$. Find the inverse function…
The monthly production cost in dollars of $x$ units of an electronic component is given by the function $C(x) = \frac{120x + 500}{x + 2}$. Find the inverse function $C^{-1}(x)$ and interpret what this new function represents in the context of the problem.
Prove algebraically that if $f(x) = \frac{x^3 + 1}{x^3 - 1}$, then its inverse function is given by the expression $f^{-1}(x) = \sqrt[3]{\frac{x+1}{x-1}}$. Next, justify…
Prove algebraically that if $f(x) = \frac{x^3 + 1}{x^3 - 1}$, then its inverse function is given by the expression $f^{-1}(x) = \sqrt[3]{\frac{x+1}{x-1}}$. Next, justify by solving an equation why the graph of $f(x)$ never intersects the graph of its inverse function.
Restricting a domain, and piecewise functions
A parabola fails the Horizontal Line Test. But if you keep only one side of the vertex, the part you keep is one-to-one, and it has an inverse.
A piecewise function can be one-to-one too. Invert each piece separately. The range of each piece becomes the domain of the matching piece of the inverse.
The quadratic function $f(x) = x^2 - 6x + 8$ does not have an inverse function in all of $\mathbb{R}$ because it is not one-to-one. Restrict its domain to the interval…
The quadratic function $f(x) = x^2 - 6x + 8$ does not have an inverse function in all of $\mathbb{R}$ because it is not one-to-one. Restrict its domain to the interval $[3, +\infty)$ and calculate the analytical expression of its inverse function in that interval.
Calculate the inverse function of the following piecewise function: $f(x) = \begin{cases} 2x + 1 & \text{if } x < 0 \\ x^2 + 1 & \text{if } x \ge 0 \end{cases}$ Prove…
Calculate the inverse function of the following piecewise function:
$f(x) = \begin{cases} 2x + 1 & \text{if } x < 0 \\ x^2 + 1 & \text{if } x \ge 0 \end{cases}$
Prove analytically that the domain of $f^{-1}$ coincides exactly with the range of $f$.
Inverses in context
In a real problem, $f$ turns one quantity into another, and $f^{-1}$ turns it back. The inverse takes the old output as its input.
The relationship that allows converting a given temperature in degrees Celsius ($C$) to degrees Fahrenheit ($F$) is $F(C) = \frac{9}{5}C + 32$. Find the inverse function…
The relationship that allows converting a given temperature in degrees Celsius ($C$) to degrees Fahrenheit ($F$) is $F(C) = \frac{9}{5}C + 32$. Find the inverse function $C(F)$ and use it to calculate how many degrees Celsius correspond to $68$ degrees Fahrenheit.
In a laboratory culture, the growth of a population of bacteria is modeled by the function $P(t) = 150 \cdot e^{0.2t}$, where $t$ is measured in hours. Determine the…
In a laboratory culture, the growth of a population of bacteria is modeled by the function $P(t) = 150 \cdot e^{0.2t}$, where $t$ is measured in hours. Determine the inverse function $t(P)$ and use it to calculate exactly how much time must elapse for the population to reach $3000$ bacteria.
The spread of a computer virus in a closed network is modeled by the logistic function $N(t) = \frac{5000}{1 + 49e^{-0.5t}}$, where $N(t)$ is the number of infected…
The spread of a computer virus in a closed network is modeled by the logistic function $N(t) = \frac{5000}{1 + 49e^{-0.5t}}$, where $N(t)$ is the number of infected computers and $t$ represents the days elapsed since the initial detection. Determine the inverse function $t(N)$. What is the meaning of the domain of this inverse function in the context of the virus's propagation limit?
Decay and half-life
Some quantities shrink over time: the value of a used car, the amount of medicine in your bloodstream, the concentration of a chemical in a reaction. A common question is "how long until only half is left?" That time is called the half-life.
To find it, you need $t$ as a function of $Q$. That is the inverse function. Then you plug in $Q = \frac{Q_0}{2}$.
The annual depreciation of industrial equipment is modeled by the function $V(t) = 12000 \cdot (0.8)^t$, where $V$ is the current value in dollars and $t$ is the time…
The annual depreciation of industrial equipment is modeled by the function $V(t) = 12000 \cdot (0.8)^t$, where $V$ is the current value in dollars and $t$ is the time elapsed in years since its purchase. Find the inverse function $t(V)$ and indicate exactly how many years have passed if the equipment is valued today at $\$6144$.
The concentration $C(t)$ of a reactant in a second-order chemical reaction evolves according to the law $C(t) = \frac{C_0}{1 + k \cdot C_0 \cdot t}$, where $C_0$ is the…
The concentration $C(t)$ of a reactant in a second-order chemical reaction evolves according to the law $C(t) = \frac{C_0}{1 + k \cdot C_0 \cdot t}$, where $C_0$ is the initial concentration and $k$ is the rate constant. Make time $t$ the subject to find the inverse function $t(C)$. Use it to prove that the half-life (the time necessary for the concentration to halve, $C = \frac{C_0}{2}$) is inversely proportional to the initial concentration $C_0$.
Values of $f^{-1}$ and self-inverse functions
Sometimes you do not need the whole formula for $f^{-1}$. You only need one value.
A function is self-inverse if $f^{-1} = f$, that is, $f(f(x)) = x$. Its graph is symmetric about $y = x$. Examples: $f(x) = -x + 3$ and $f(x) = \frac{1}{x}$.
Knowing that $f(x) = x^3 + 2x + 1$, calculate the numerical value of $f^{-1}(4)$ without needing to find the complete algebraic expression of the inverse function.
Knowing that $f(x) = x^3 + 2x + 1$, calculate the numerical value of $f^{-1}(4)$ without needing to find the complete algebraic expression of the inverse function.
Find the value of the parameter $k$ so that the function $f(x) = \frac{3x + 2}{x + k}$ is its own inverse, that is, that the condition $f(f(x)) = x$ is fulfilled for any…
Find the value of the parameter $k$ so that the function $f(x) = \frac{3x + 2}{x + k}$ is its own inverse, that is, that the condition $f(f(x)) = x$ is fulfilled for any value in its domain.
Knowing that the rational function $f(x) = \frac{ax + b}{cx - 1}$ is its own inverse function (that is, $f^{-1}(x) = f(x)$) and that its graph passes through the point…
Knowing that the rational function $f(x) = \frac{ax + b}{cx - 1}$ is its own inverse function (that is, $f^{-1}(x) = f(x)$) and that its graph passes through the point $(2, 5)$, set up a system of equations and determine the numerical values of the parameters $a$, $b$ and $c$.
Where $f$ meets $f^{-1}$, and inverses of compositions
Because $f$ and $f^{-1}$ are mirror images across $y = x$, any point of $f$ that lies on the line $y = x$ is shared with $f^{-1}$.
Now the inverse of a composition. In $(f \circ g)(x) = f(g(x))$ the function $g$ acts first and $f$ second. To undo it, undo $f$ first and then $g$, like taking off your shoes before your socks if you put the socks on first.
The graph of the function $f(x) = x^2 - 2$ restricted to the domain $x \ge 0$ is shown. Draw the line $y = x$ and the graph of the inverse function $f^{-1}(x)$ in your…
The graph of the function $f(x) = x^2 - 2$ restricted to the domain $x \ge 0$ is shown. Draw the line $y = x$ and the graph of the inverse function $f^{-1}(x)$ in your notebook. Analytically determine the exact point of intersection between both curves.
Let the radical function be $f(x) = \sqrt{x+6}$. Calculate its inverse function $f^{-1}(x)$, clearly indicating its domain. Analytically determine the points of…
Let the radical function be $f(x) = \sqrt{x+6}$. Calculate its inverse function $f^{-1}(x)$, clearly indicating its domain. Analytically determine the points of intersection between the graphs of $f(x)$ and $f^{-1}(x)$, knowing that by symmetry these points must lie on the line $y = x$.
Given the functions $f(x) = \frac{x}{x-2}$ and $g(x) = \sqrt{x+1}$, obtain step by step the analytical expression of the inverse function of the composition, that is,…
Given the functions $f(x) = \frac{x}{x-2}$ and $g(x) = \sqrt{x+1}$, obtain step by step the analytical expression of the inverse function of the composition, that is, $(f \circ g)^{-1}(x)$, and determine the domain of validity of this new function.
Back to the beginning: to turn Fahrenheit into Celsius, solve $F = \tfrac{9}{5}C + 32$ for $C$: subtract 32, then multiply by $\tfrac{5}{9}$. That gives $C = \tfrac{5}{9}(F - 32)$. For $F = 86$: $C = \tfrac{5}{9}(54) = 30$, so Phoenix's high of 86 °F is 30 °C.