3.7 Inverse functions (Page 5/9)

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Using the graph in [link] , (a) find $g^{- 1} (1),$ and (b) estimate $g^{- 1} (4) .$

a. 3; b. 5.6

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Finding inverses of functions represented by formulas

Sometimes we will need to know an inverse function for all elements of its domain, not just a few. If the original function is given as a formula—for example, $y$ as a function of $x —$ we can often find the inverse function by solving to obtain $x$ as a function of $y .$

How To

Given a function represented by a formula, find the inverse.

Make sure $f$ is a one-to-one function.
Solve for $x .$
Interchange $x$ and $y .$

Inverting the fahrenheit-to-celsius function

Find a formula for the inverse function that gives Fahrenheit temperature as a function of Celsius temperature.

C = \frac{5}{9} (F - 32)

\begin{matrix} C & = & \frac{5}{9} (F - 32) \\ C \cdot \frac{9}{5} & = & F - 32 \\ F & = & \frac{9}{5} C + 32 \end{matrix}

By solving in general, we have uncovered the inverse function. If

C = h (F) = \frac{5}{9} (F - 32),

then

F = h^{- 1} (C) = \frac{9}{5} C + 32

In this case, we introduced a function $h$ to represent the conversion because the input and output variables are descriptive, and writing $C^{- 1}$ could get confusing.

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Solve for $x$ in terms of $y$ given $y = \frac{1}{3} (x - 5) .$

$x = 3 y + 5$

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Solving to find an inverse function

Find the inverse of the function $f (x) = \frac{2}{x - 3} + 4.$

\begin{matrix} y & = & \frac{2}{x - 3} + 4 & Set up an equation . \\ y - 4 & = & \frac{2}{x - 3} & Subtract 4 from both sides . \\ x - 3 & = & \frac{2}{y - 4} & Multiply both sides by x - 3 and divide by y - 4. \\ x & = & \frac{2}{y - 4} + 3 & Add 3 to both sides . \end{matrix}

So $f^{- 1} (y) = \frac{2}{y - 4} + 3$ or $f^{- 1} (x) = \frac{2}{x - 4} + 3.$

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Solving to find an inverse with radicals

Find the inverse of the function $f (x) = 2 + \sqrt{x - 4} .$

\begin{matrix} y & = & 2 + \sqrt{x - 4} \\ {(y - 2)}^{2} & = & x - 4 \\ x & = & {(y - 2)}^{2} + 4 \end{matrix}

So $f^{- 1} (x) = {(x - 2)}^{2} + 4.$

The domain of $f$ is $[4, \infty) .$ Notice that the range of $f$ is $[2, \infty),$ so this means that the domain of the inverse function $f^{- 1}$ is also $[2, \infty) .$

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What is the inverse of the function $f (x) = 2 - \sqrt{x} ?$ State the domains of both the function and the inverse function.

$f^{- 1} (x) = {(2 - x)}^{2}; domain of f : [0, \infty); domain of f^{- 1} : (- \infty, 2]$

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Finding inverse functions and their graphs

Now that we can find the inverse of a function, we will explore the graphs of functions and their inverses. Let us return to the quadratic function $f (x) = x^{2}$ restricted to the domain $[0, \infty),$ on which this function is one-to-one, and graph it as in [link] .

Graph of f(x). — Quadratic function with domain restricted to [0, ∞).

Restricting the domain to $[0, \infty)$ makes the function one-to-one (it will obviously pass the horizontal line test), so it has an inverse on this restricted domain.

We already know that the inverse of the toolkit quadratic function is the square root function, that is, $f^{- 1} (x) = \sqrt{x} .$ What happens if we graph both $f$ and $f^{- 1}$ on the same set of axes, using the $x -$ axis for the input to both $f and f^{- 1} ?$

We notice a distinct relationship: The graph of $f^{- 1} (x)$ is the graph of $f (x)$ reflected about the diagonal line $y = x,$ which we will call the identity line, shown in [link] .

Graph of f(x) and f^(-1)(x). — Square and square-root functions on the non-negative domain

This relationship will be observed for all one-to-one functions, because it is a result of the function and its inverse swapping inputs and outputs. This is equivalent to interchanging the roles of the vertical and horizontal axes.

Finding the inverse of a function using reflection about the identity line

Given the graph of $f (x)$ in [link] , sketch a graph of $f^{- 1} (x) .$

This is a one-to-one function, so we will be able to sketch an inverse. Note that the graph shown has an apparent domain of $(0, \infty)$ and range of $(- \infty, \infty),$ so the inverse will have a domain of $(- \infty, \infty)$ and range of $(0, \infty) .$

If we reflect this graph over the line $y = x,$ the point $(1, 0)$ reflects to $(0, 1)$ and the point $(4, 2)$ reflects to $(2, 4) .$ Sketching the inverse on the same axes as the original graph gives [link] .

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Questions & Answers

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When MP₁ becomes negative, TP start to decline. Extuples Suppose that the short-run production function of certain cut-flower firm is given by: Q=4KL-0.6K2 - 0.112 • Where is quantity of cut flower produced, I is labour input and K is fixed capital input (K-5). Determine the average product of lab

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Extuples Suppose that the short-run production function of certain cut-flower firm is given by: Q=4KL-0.6K2 - 0.112 • Where is quantity of cut flower produced, I is labour input and K is fixed capital input (K-5). Determine the average product of labour (APL) and marginal product of labour (MPL)

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What is different between quantity demand and demand?

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In economics, the contract curve refers to the set of points in an Edgeworth box diagram where both parties involved in a trade cannot be made better off without making one of them worse off. It represents the Pareto efficient allocations of goods between two individuals or entities, where neither p

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Suppose a consumer consuming two commodities X and Y has The following utility function u=X0.4 Y0.6. If the price of the X and Y are 2 and 3 respectively and income Constraint is birr 50. A,Calculate quantities of x and y which maximize utility. B,Calculate value of Lagrange multiplier. C,Calculate quantities of X and Y consumed with a given price. D,alculate optimum level of output .

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Answer

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the market for lemon has 10 potential consumers, each having an individual demand curve p=101-10Qi, where p is price in dollar's per cup and Qi is the number of cups demanded per week by the i th consumer.Find the market demand curve using algebra. Draw an individual demand curve and the market dema

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suppose the production function is given by ( L, K)=L¼K¾.assuming capital is fixed find APL and MPL. consider the following short run production function:Q=6L²-0.4L³ a) find the value of L that maximizes output b)find the value of L that maximizes marginal product

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Source: OpenStax, Algebra and trigonometry. OpenStax CNX. Nov 14, 2016 Download for free at https://legacy.cnx.org/content/col11758/1.6

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