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Secondary 5 · Mathematics
Functions · Real functions

Inverse function

Before you start, make sure you know: First-degree equations (Secondary Year 2 (first cycle))

Yaritza is traveling to the United States and sees in the forecast that it will be 68 degrees Fahrenheit. In Santo Domingo people always talk about degrees Celsius. To go from Celsius to Fahrenheit there is a formula; to go the other way you need another one that "undoes" exactly what the first one did. That function is the inverse function. How do you find it?

Injective function: the condition for having an inverse

Graph of a parabola crossed by a horizontal line at two points with the same y-value, showing that it is not injective. same y, two x: not injective
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The quadratic function $f(x) = x^2 - 6x + 8$ does not have an inverse function on all of $\mathbb{R}$ because it is not injective. Restrict its domain to the interval…

The quadratic function $f(x) = x^2 - 6x + 8$ does not have an inverse function on all of $\mathbb{R}$ because it is not injective. Restrict its domain to the interval $[3, +\infty)$ and calculate the analytical expression for its inverse function on that interval.

Solve

Finding the inverse function

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Find the analytical expression for the inverse function of the affine function $f(x) = 3x - 5$.

Find the analytical expression for the inverse function of the affine function $f(x) = 3x - 5$.

Solve
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Determine the inverse function of the rational function $g(x) = \frac{4}{x+2}$.

Determine the inverse function of the rational function $g(x) = \frac{4}{x+2}$.

Solve
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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.

Solve

The value of $f^{-1}(a)$ without finding the inverse

Sometimes you don't need the complete expression for $f^{-1}$, only its value at one number. There is a shortcut.

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Knowing that $f(x) = x^3 + 2x + 1$, calculate the numerical value of $f^{-1}(4)$ without needing to find the full 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 full algebraic expression of the inverse function.

Solve

Inverses of exponentials and logarithms

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Obtain the inverse function of the exponential function $f(x) = 2^{x-3}$.

Obtain the inverse function of the exponential function $f(x) = 2^{x-3}$.

Solve
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Find the inverse function of the logarithmic function $g(x) = \log_3(x) + 1$.

Find the inverse function of the logarithmic function $g(x) = \log_3(x) + 1$.

Solve
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The number of users of a new messaging app grows according to the model $U(t) = 150 \times e^{0.2t}$, where $t$ is measured in weeks and $U(t)$ in thousands of users.…

The number of users of a new messaging app grows according to the model $U(t) = 150 \times e^{0.2t}$, where $t$ is measured in weeks and $U(t)$ in thousands of users. Determine the inverse function $t(U)$ and use it to calculate the exact time that must elapse for the app to reach $3000$ thousand users (three million).

Solve
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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 sound 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 decibels. If the human pain threshold is at $120 \text{ dB}$, what is the corresponding sound intensity?

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Checking an inverse by composition

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Using function composition, check 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…

Using function composition, check 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 $(g \circ f)(x) = x$.

Solve
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Find the value of the parameter $k$ such that the function $f(x) = \frac{3x + 2}{x + k}$ is its own inverse, that is, satisfying the condition $f(f(x)) = x$ for any…

Find the value of the parameter $k$ such that the function $f(x) = \frac{3x + 2}{x + k}$ is its own inverse, that is, satisfying the condition $f(f(x)) = x$ for any value in its domain.

Solve
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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 points…

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 points $(2, 5)$ and $(0, -3)$, set up a system of equations and determine the numerical values of the parameters $a$, $b$, and $c$.

Solve

Symmetry about $y=x$, and the swapped domain and range

Graphs of a function f and its inverse, symmetric about the dashed line y = x. f f⁻¹ y = x
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Given the graph of the exponential function $f(x) = 2^x$, draw the coordinate axes and the line $y = x$ in your exercise book, and sketch the graph of its inverse…

Given the graph of the exponential function $f(x) = 2^x$, draw the coordinate axes and the line $y = x$ in your exercise book, 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.

X Y 1 2 4 f(x) = 2ˣ

You can do it on paper and send a photo with your answer.

Solve
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Given the rational function $f(x) = \frac{2x - 1}{x + 3}$, find its inverse function $f^{-1}(x)$. Then, 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 $f^{-1}(x)$. Then, calculate the domain of $f(x)$ and the domain of $f^{-1}(x)$, and explain the relationship between these domains and the ranges of both functions.

Solve
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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 exercise book. Determine analytically the exact point of intersection between both curves.

X Y −2 1 2 f(x) = x² - 2

You can do it on paper and send a photo with your answer.

Solve
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Let $f(x) = \sqrt{x+6}$ be the radical function. Calculate its inverse function $f^{-1}(x)$, clearly stating its domain. Analytically determine the points of…

Let $f(x) = \sqrt{x+6}$ be the radical function. Calculate its inverse function $f^{-1}(x)$, clearly stating 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 $y = x$.

X Y f(x) f⁻¹(x) ?
Solve

Word problems and special cases

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The monthly production cost in pesos of $x$ units of an electronic component is given by the function $C(x) = \frac{7\,200x + 30\,000}{x + 2}$. Find the inverse function…

The monthly production cost in pesos of $x$ units of an electronic component is given by the function $C(x) = \frac{7\,200x + 30\,000}{x + 2}$. Find the inverse function $C^{-1}(x)$ and interpret what this new function represents in the context of the problem.

Solve
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The relationship used to convert a temperature given in degrees Celsius ($C$) to degrees Fahrenheit ($F$) is $F(C) = \frac{9}{5}C + 32$. Find the inverse function $C(F)$…

The relationship used to convert a temperature given 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.

Solve
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The annual depreciation of industrial equipment is modeled by the function $V(t) = 720\,000 \times (0,8)^t$, where $V$ is the current value in pesos and $t$ is the time…

The annual depreciation of industrial equipment is modeled by the function $V(t) = 720\,000 \times (0,8)^t$, where $V$ is the current value in pesos and $t$ is the time elapsed in years since its purchase. Find the inverse function $t(V)$ and state how many years have passed exactly if the equipment is valued today at $368\,640$ pesos.

Solve
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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 composite function, that…

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 composite function, that is, $(f \circ g)^{-1}(x)$, and determine the domain of validity of this new function.

Solve
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Calculate the inverse function of the following piecewise function: $f(x) = \begin{cases} 2x + 1 & \text{if } x < 0 \\ x^2 + 1 & \text{if } x \ge 0 \end{cases}$ Verify…

Calculate the inverse function of the following piecewise function:

$f(x) = \begin{cases} 2x + 1 & \text{if } x < 0 \\ x^2 + 1 & \text{if } x \ge 0 \end{cases}$

Verify analytically that the domain of $f^{-1}$ coincides exactly with the range of $f$.

Solve
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The concentration $C(t)$ of a reactant in a second-order chemical reaction evolves according to the law $C(t) = \frac{C_0}{1 + k \times C_0 \times t}$, where $C_0$ is…

The concentration $C(t)$ of a reactant in a second-order chemical reaction evolves according to the law $C(t) = \frac{C_0}{1 + k \times C_0 \times t}$, where $C_0$ is the initial concentration and $k$ is the rate constant. Rearrange for time $t$ to find the inverse function $t(C)$. Use it to prove that the half-life (the time required for the concentration to be halved, $C = \frac{C_0}{2}$) is inversely proportional to the initial concentration $C_0$.

Solve
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The spread of a computer virus in a closed network is modelled using 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 modelled using 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 initial detection. Determine the inverse function $t(N)$. What meaning does the domain of this inverse function have in the context of the limit of propagation of the virus?

Solve
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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 whether the graph of $f(x)$ intersects the graph of its inverse function.

Solve

Back to the beginning: to go from Fahrenheit to Celsius you solve for $C$ in $F=\frac{9}{5}C+32$: subtract 32 and multiply by $\frac{5}{9}$, giving $C=\frac{5}{9}(F-32)$. So 68 °F is 20 °C.

Summary

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