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Functions · Real functions

Inverse function

To convert degrees Celsius to Fahrenheit there is a formula; to go the other way, from Fahrenheit to Celsius, there is another one, which "undoes" exactly what the first one did. A function that undoes another function is its inverse function, a new formal concept in Bachillerato built directly on composition.

Before you start, make sure you know: Basic concepts (4th ESO)
Before you start, make sure you know: First-degree equations (2nd ESO)

Injective function: the condition for having an inverse

Graph of a parabola cut by a horizontal line at two different points with the same y value, showing that it is not injective. same y, two x values: 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

Calculating 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

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

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

Animation of the graph of a function f and its inverse f⁻¹, symmetrical about the line y = x: a point moves along f, its reflection moves along f⁻¹ and a dashed segment joins them, crossing the line y = x, with the coordinates swapped. f f⁻¹
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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

Back to the start: If the formula for converting degrees Celsius to Fahrenheit is $F = \frac{9}{5}C + 32$, to find the inverse function we make $C$ the subject: we subtract $32$ and multiply by $\frac{5}{9}$, obtaining $C = \frac{5}{9}(F - 32)$.

Summary

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