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(a) The inverse of \( f(x) = x^2 \) is \( y = \pm \sqrt{x} \).
(b) The graph of \( f(x) = x^2 \) is a parabola opening upwards. Its inverse consists of two branches: \( y = \sqrt{x} \) and \( y = -\sqrt{x} \), which are reflections of the original graph across the line \( y = x \).
(c) The inverse is not a function because for each \( x > 0 \), there are two outputs: \( \sqrt{x} \) and \( -\sqrt{x} \).
(d) By restricting the domain to \( x \geq 0 \) or \( x \leq 0 \), \( f(x) = x^2 \) becomes one-to-one.
(e) For \( x \geq 0 \), the inverse function is \( f^{-1}(x) = \sqrt{x} \). For \( x \leq 0 \), the inverse function is \( f^{-1}(x) = -\sqrt{x} \).
(f) Domain and range for each inverse function:
- For \( f^{-1}(x) = \sqrt{x} \): Domain \( [0, \infty) \), Range \( [0, \infty) \).
- For \( f^{-1}(x) = -\sqrt{x} \): Domain \( [0, \infty) \), Range \( (-\infty, 0] \).
Solución
In this exercise we are meant to start with the function
f(x) = x²
and investigate its inverse. (Note that if no function is given, it is common in such problems that f is taken to be x².) Let’s go through each part.
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(a) Write down the inverse of f(x) in the form y = …
Step 1. Write f in “y=” form:
y = x²
Step 2. To “find the inverse” we interchange the roles of x and y:
x = y²
Step 3. Solve for y. Taking square roots we have
y = √x or y = –√x.
Thus, the inverse relation is
y = ±√x.
Note: Since the original f(x) = x² is not one‑to‑one (because, for example, f(2) = f(–2) = 4), its inverse “relation” is given by these two branches.
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(b) Sketch the graph of the function and its inverse on the same set of axes.
To sketch:
• The graph of f(x) = x² is a parabola opening upward with vertex at (0, 0).
• The “graph” of the inverse consists of the two halves of the sideways parabola:
y = √x (the upper half) and y = –√x (the lower half).
• Notice that the graph of the inverse relation is the mirror image of f(x) about the line y = x.
(When you sketch, draw the parabola along with the line y = x and then reflect the curve across this line to get the inverse relation.)
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(c) Is the inverse a function? Justify your answer.
No, the inverse is not a function. For a relation to be a function it must assign exactly one output value to each input. Here, for a given x (with x > 0) the inverse relation gives two outputs: one positive and one negative (√x and –√x). Equivalently, the graph of y = ±√x fails the horizontal line test. Therefore, the inverse of f(x) = x² (with its natural domain) is not a function.
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(d) Restrict the domain of the given function in two different ways to form a one‐to‐one function.
Since f(x) = x² is not one‐to‐one on ℝ, we can restrict its domain to either of the following:
1. Restriction 1: Let f be defined on [0, ∞). That is, define
f₁(x) = x² for x ≥ 0.
2. Restriction 2: Alternatively, let f be defined on (–∞, 0]. That is, define
f₂(x) = x² for x ≤ 0.
Both f₁ and f₂ are one‐to‐one.
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(e) Sketch the graphs of each new function and its inverse on the same set of axes.
For the first restricted function, f₁(x) = x², x ≥ 0:
• f₁(x) is the right half of the parabola.
• Its inverse is obtained by solving x = y² with y ≥ 0. This gives
f₁⁻¹(x) = √x.
• Sketch f₁(x) on the x ≥ 0 portion and the graph of y = √x; note that these graphs are reflections of each other in the line y = x.
For the second restricted function, f₂(x) = x², x ≤ 0:
• f₂(x) is the left half of the parabola.
• Its inverse is obtained by solving x = y² with y ≤ 0. This gives
f₂⁻¹(x) = –√x.
• Similarly, when you graph f₂ and y = –√x, the two graphs are reflections of each other in the line y = x.
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(f) Rewrite the equation of each inverse function in the form f⁻¹(x).
Thus we have:
1. For f₁(x) = x² with x ≥ 0, the inverse function is
f₁⁻¹(x) = √x (with x ≥ 0)
2. For f₂(x) = x² with x ≤ 0, the inverse function is
f₂⁻¹(x) = –√x (with x ≥ 0)
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(g) Write down the domain and range of each main graph.
For the original f(x) = x² (without restriction):
• Domain: all real numbers, ℝ.
• Range: [0, ∞).
• Its inverse “relation” (y = ±√x) has domain [0, ∞) and range (–∞, ∞), but note that it is not a function.
For the restricted functions:
1. For f₁:
• Domain of f₁: [0, ∞)
• Range of f₁: [0, ∞)
• Domain of f₁⁻¹ (i.e. f₁⁻¹(x) = √x): [0, ∞)
• Range of f₁⁻¹: [0, ∞)
2. For f₂:
• Domain of f₂: (–∞, 0]
• Range of f₂: [0, ∞) (since squaring any number is nonnegative)
• Domain of f₂⁻¹ (i.e. f₂⁻¹(x) = –√x): [0, ∞)
• Range of f₂⁻¹: (–∞, 0] (because –√x is never positive)
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Summary
(a) The inverse relation of f(x)=x² is: y = ±√x.
(b) The graph of y = x² (a parabola) and the graph of its inverse (the two branches y = √x and y = –√x) are reflections of each other in the line y = x.
(c) The inverse relation is not a function (since, for most x, there are two y‐values).
(d) By restricting the domain to [0, ∞) or to (–∞, 0], f becomes one-to-one.
(e) The graphs of
f₁(x)=x² (x ≥ 0) with inverse f₁⁻¹(x)=√x, and
f₂(x)=x² (x ≤ 0) with inverse f₂⁻¹(x)= –√x
are each symmetric about y = x.
(f) Inverse functions in proper notation:
For x ≥ 0: f₁⁻¹(x) = √x.
For x ≤ 0: f₂⁻¹(x) = –√x.
(g) Their domains and ranges are as stated above.
Any answer equivalent to the one given is correct.
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