Restrict Domain to Make Invertible
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Algebra 2 › Restrict Domain to Make Invertible
For $f(x)=(x-1)^2$ with restricted domain $x \ge 1$ (so it passes the horizontal line test), what is $f^{-1}(x)$?
$f^{-1}(x)=\sqrt{x}-1$
$f^{-1}(x)=1+\sqrt{x}$
$f^{-1}(x)=\pm\sqrt{x}+1$
$f^{-1}(x)=1-\sqrt{x}$
Explanation
This question tests your understanding that some functions (like $f(x) = (x-1)^2$) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For $f(x) = (x-1)^2$ restricted to $x \ge 1$, it's increasing from 0 to $\infty$. To find the inverse, set $y = (x-1)^2$ ($x \ge 1$), swap to $x = (y-1)^2$, solve $y-1 = \sqrt{x}$ (since $y \ge 1$ implies $y-1 \ge 0$), so $y = 1 + \sqrt{x}$. This matches the restricted range. For even-degree polynomials like $f(x) = x^2$ or $f(x) = (x - 3)^2 + 1$, standard restrictions are to the right or left of the vertex: restrict to $x \ge h$ or $x \le h$ where $h$ is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. For $(x-1)^2$, restricting to $x \ge 1$ gives the positive branch. Choice C correctly finds $f^{-1}(x) = 1 + \sqrt{x}$. Choice D fails because $\pm$ would not be a single-valued function. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for $x^2$, use $x \ge 0$; for $(x - h)^2 + k$, use $x \ge h$ or $x \le h$; for $x^4$, use $x \ge 0$. The restricted domain should be an interval $[a, \infty)$ or $(-\infty, a]$ where the function is one-to-one. After restricting, you can find the inverse: with $f(x) = (x-1)^2$ restricted to $x \ge 1$, the positive root ensures it works. Fantastic progress on finding inverses!
Which domain restriction makes $f(x)=x^2-6x+5$ pass the horizontal line test (and therefore be invertible)?
Domain: $x\le 3$
Domain: $[1,\infty)$
Domain: $x\le 0$
Domain: $(-\infty,\infty)$
Explanation
This question tests your understanding that some functions (like f(x) = x² -6x +5) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = x² -6x +5 = (x-3)² -4, vertex at x=3, it fails on all reals because f(1)=0 and f(5)=0. But restricting to x ≤ 3 makes it decreasing from ∞ to -4, passing the test. The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. Here, x ≤ 3 ensures decreasing. Choice A correctly restricts to x ≤ 3 to make it pass the horizontal line test. Choice D fails because [1,∞) spans both sides (f(1)=0, f(5)=0), not one-to-one. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. You're nailing these vertex identifications!
The function $f(x)=-x^2+10$ is not invertible on all real numbers because it fails the horizontal line test. Which restriction makes $f$ invertible?
Domain: $-1\le x\le 1$
Domain: $x\ge 0$
Domain: $(-\infty,\infty)$
Domain: $x\ne 0$
Explanation
This question tests your understanding that some functions (like f(x) = -x² +10) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = -x² +10, a downward parabola with vertex at x=0, it fails on all reals because f(-1)=9 and f(1)=9. But restricting to x ≥ 0 makes it decreasing from 10 to -∞, passing the test. The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. Here, for a downward parabola, x ≥ 0 ensures decreasing monotonicity. Choice B correctly restricts to x ≥ 0 to make it invertible. Choice C fails because [-1,1] includes both sides (f(-1)=9, f(1)=9), not one-to-one. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. Keep exploring these downward parabolas—you're doing wonderfully!
The function $f(x)=x^2-4x+3$ fails the horizontal line test on all real numbers because it is a parabola. Which restriction makes $f$ invertible?
Domain: $[2,\infty)$
Domain: $(-\infty,1]$
Domain: $[0,\infty)$
Domain: $(-\infty,\infty)$
Explanation
This question tests your understanding that some functions (like f(x) = x² -4x +3) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = x² -4x +3, which is (x-2)² -1 with vertex at x=2, it's not one-to-one on all reals because f(1)=0 and f(3)=0. But if we restrict to x ≥ 2 (right of the vertex), the function is increasing from -1 to ∞, so every horizontal line above -1 crosses once—now it's invertible! The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. Here, [2,∞) ensures monotonic increase. Choice A correctly restricts the domain to [2,∞) to make it one-to-one and invertible. Choice C fails because [0,∞) spans both sides of the vertex (f(0)=3, f(4)=3), violating one-to-one. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. After restricting, you can find the inverse by solving y = (x-2)² -1 for x in terms of y, taking the positive branch since x ≥ 2. Great job spotting the vertex—you've got this!
Explain why $f(x)=x^2$ (domain: all real numbers) needs a domain restriction to have an inverse. Which statement correctly uses the horizontal line test idea?
It fails the horizontal line test because some horizontal lines intersect the graph more than once, so the inverse would not be a function.
It has an inverse on all real numbers because it is a polynomial.
It fails the vertical line test, so it cannot have an inverse.
It passes the horizontal line test because every horizontal line intersects the graph exactly twice.
Explanation
This question tests your understanding that some functions (like f(x) = x²) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = x², the horizontal line y = 4 crosses at both x = 2 and x = -2, so x² isn't one-to-one on all reals. But if we restrict to x ≥ 0 (right half only), every horizontal line crosses at most once—now it's invertible! The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. For x², restricting to x ≥ 0 gives the 'standard' inverse f⁻¹(x) = √x (principal square root). Restricting to x ≤ 0 would give f⁻¹(x) = -√x instead. The function fails without restriction because the inverse relation would map y to two x's, like 4 to ±2, which isn't a function. Choice B correctly explains that it fails the horizontal line test because some lines intersect more than once, so the inverse isn't a function. Choice A is incorrect because intersecting exactly twice means it fails, not passes, the test. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. After restricting, you can find the inverse: with f(x) = x² restricted to x ≥ 0, swap and solve: y = x², swap to x = y², solve for y = √x (taking positive root because we restricted to x ≥ 0!). The domain restriction affects which branch of the inverse you get. No restriction means you can't choose between √x and -√x—both would be needed, but that's not a function. Restriction lets you pick one branch! You're mastering the horizontal line test—keep it up!
A student claims $f(x)=x^2$ is invertible on all real numbers because you can “solve” $y=x^2$ by writing $x=\pm\sqrt{y}$. Which domain restriction correctly fixes the issue so the inverse is a function (passes the horizontal line test)?
Restrict the domain to $-1\le x\le 1$, so the inverse is $f^{-1}(x)=\pm\sqrt{x}$.
Restrict the domain to $x\ge 0$, so the inverse is $f^{-1}(x)=\sqrt{x}$.
Restrict the domain to $x\ne 0$, so the inverse is $f^{-1}(x)=\sqrt{x}$.
Restrict the domain to all real numbers, so the inverse is $f^{-1}(x)=\pm\sqrt{x}$.
Explanation
This question tests your understanding that some functions (like f(x) = x²) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = x², the horizontal line y = 4 crosses at both x = 2 and x = -2, so x² isn't one-to-one on all reals. But if we restrict to x ≥ 0 (right half only), every horizontal line crosses at most once—now it's invertible! The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. For x², restricting to x ≥ 0 gives the 'standard' inverse f⁻¹(x) = √x (principal square root). Restricting to x ≤ 0 would give f⁻¹(x) = -√x instead. The student's claim is wrong because ±√y is a relation, not a function, as it gives two outputs for one input. Choice A correctly restricts to x ≥ 0 so the inverse is the function √x. Choice B fails because on all reals, the inverse isn't a function due to multiple values. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. After restricting, you can find the inverse: with f(x) = x² restricted to x ≥ 0, swap and solve: y = x², swap to x = y², solve for y = √x (taking positive root because we restricted to x ≥ 0!). The domain restriction affects which branch of the inverse you get. No restriction means you can't choose between √x and -√x—both would be needed, but that's not a function. Restriction lets you pick one branch! You're correcting misconceptions like a pro—great job!
After restricting the domain of $f(x)=x^2$ to $x \geq 0$ so it passes the horizontal line test, what is the inverse function $f^{-1}(x)$?
$f^{-1}(x) = x^2$
$f^{-1}(x) = \sqrt{x}$
$f^{-1}(x) = \pm \sqrt{x}$
$f^{-1}(x) = \sqrt{x-0}$ with domain $(-\infty, \infty)$
Explanation
This question tests your understanding that some functions (like $f(x) = x^2$) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For $f(x) = x^2$ restricted to $x \geq 0$, it passes the test as it's strictly increasing from 0 to $\infty$. To find the inverse, set $y = x^2$ ($x \geq 0$), swap to $x = y^2$, and solve $y = \sqrt{x}$ (principal square root, since $y \geq 0$ matches the range). This gives a proper function inverse. For even-degree polynomials like $f(x) = x^2$ or $f(x) = (x - 3)^2 + 1$, standard restrictions are to the right or left of the vertex: restrict to $x \geq h$ or $x \leq h$ where $h$ is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. For $x^2$, restricting to $x \geq 0$ gives the 'standard' inverse $f^{-1}(x) = \sqrt{x}$ (principal square root). Restricting to $x \leq 0$ would give $f^{-1}(x) = -\sqrt{x}$ instead. Choice B correctly finds the inverse as $f^{-1}(x) = \sqrt{x}$ after the restriction. Choice A fails because $\pm \sqrt{x}$ is not a single function—it maps to two values, violating the definition of a function. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for $x^2$, use $x \geq 0$; for $(x - h)^2 + k$, use $x \geq h$ or $x \leq h$; for $x^4$, use $x \geq 0$. The restricted domain should be an interval $[a, \infty)$ or $(-\infty, a]$ where the function is one-to-one. After restricting, you can find the inverse: with $f(x) = x^2$ restricted to $x \geq 0$, swap and solve: $y = x^2$, swap to $x = y^2$, solve for $y = \sqrt{x}$ (taking positive root because we restricted to $x \geq 0$). The domain restriction affects which branch of the inverse you get. No restriction means you can't choose between $\sqrt{x}$ and $-\sqrt{x}$—both would be needed, but that's not a function. Restriction lets you pick one branch! Excellent work on inverses—keep going!
Restrict the domain of $f(x)=(x-3)^2+1$ (domain: all real numbers) so that it passes the horizontal line test and is invertible. Which domain restriction works?
Domain: $(-\infty,\infty)$
Domain: $(-\infty,3]$
Domain: $(-\infty,1]$
Domain: $[0,\infty)$
Explanation
This question tests your understanding that some functions (like f(x) = (x-3)² +1) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = (x-3)² +1, the vertex is at x=3, and it's not one-to-one on all reals because, for example, f(2)= (2-3)² +1=2 and f(4)= (4-3)² +1=2. But if we restrict to x ≤ 3 (left of the vertex), the function is decreasing, so every horizontal line crosses at most once—now it's invertible! The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. For (x-3)² +1, restricting to x ≤ 3 gives a decreasing function. The parabola opens upward with minimum at (3,1), so on (-∞,3], it decreases from ∞ to 1, ensuring no repeats. Choice B correctly restricts the domain to (-∞,3] to make it one-to-one and invertible. Choice D fails because [0,∞) includes points on both sides of the vertex (like x=0 and x=6, where f(0)= (0-3)² +1=10 and f(6)= (6-3)² +1=10), so not one-to-one. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. After restricting, you can find the inverse: with f(x) = (x-3)² +1 restricted to x ≤ 3, you'd solve accordingly, taking the appropriate branch. You're doing great—keep identifying those vertices!
The function $f(x)=(x+1)^2$ is not one-to-one on all real numbers (it fails the horizontal line test). Which domain restriction makes it invertible?
Domain: $-2\le x\le 2$
Domain: $x\ge 1$
Domain: $(-\infty,\infty)$
Domain: $x\ge -1$
Explanation
This question tests your understanding that some functions (like f(x) = (x+1)²) aren't invertible on their full domain because they're not one-to-one, but we can make them invertible by restricting the domain to a region where they pass the horizontal line test. A function must be one-to-one (each output comes from exactly one input) to have an inverse, which we verify with the horizontal line test: if any horizontal line crosses the graph more than once, the function isn't one-to-one. For f(x) = (x+1)², vertex at x=-1, it fails on all reals because f(-2)=1 and f(0)=1. But restricting to x ≥ -1 makes it increasing from 0 to ∞, passing the test. The restriction creates one-to-one behavior. For even-degree polynomials like f(x) = x² or f(x) = (x - 3)² + 1, standard restrictions are to the right or left of the vertex: restrict to x ≥ h or x ≤ h where h is the vertex's x-coordinate. This makes the function monotonic (always increasing or always decreasing), which guarantees one-to-one. Here, x ≥ -1 ensures monotonicity. Choice A correctly restricts to x ≥ -1 to make it invertible. Choice D fails because [-2,2] includes both sides (f(-2)=1, f(0)=1), not one-to-one. Domain restriction decision tree: (1) Graph the function (or visualize it), (2) Find where it fails horizontal line test (usually at vertex for parabolas), (3) Choose a domain making the function monotonic—either the increasing part or the decreasing part, (4) Standard choices: for x², use x ≥ 0; for (x - h)² + k, use x ≥ h or x ≤ h; for x⁴, use x ≥ 0. The restricted domain should be an interval [a, ∞) or (-∞, a] where the function is one-to-one. You're building strong skills here!
After restricting the domain of $f(x)=x^2$ to $x \ge 0$ so it passes the horizontal line test, what is the inverse function $f^{-1}(x)$?
$f^{-1}(x)=-\sqrt{x}$
$f^{-1}(x)=x^2$
$f^{-1}(x)=\sqrt{x}$
$f^{-1}(x)=\pm\sqrt{x}$
Explanation
This question builds on restricting domains to make functions invertible, focusing now on finding the inverse after the restriction—great job getting this far! For even-degree polynomials like $f(x) = x^2$, standard restrictions are to the right or left of the vertex: restrict to $x \ge h$ or $x \le h$ where $h$ is the vertex's x-coordinate, making the function monotonic and one-to-one. For $x^2$ restricted to $x \ge 0$, this gives the 'standard' inverse $f^{-1}(x) = \sqrt{x}$ (principal square root), while restricting to $x \le 0$ would give $f^{-1}(x) = -\sqrt{x}$ instead. To find the inverse, set $y = x^2$ with $x \ge 0$, swap to $x = y^2$, and solve for $y = \sqrt{x}$, taking the non-negative root because the restricted domain ensures $y \ge 0$. Choice A correctly gives $f^{-1}(x) = \sqrt{x}$, matching the positive branch selected by the $x \ge 0$ restriction. Choice B includes $\pm$, but the inverse must be a function, so we can't have both branches; C is the original function, and D takes the negative root, which wouldn't match the domain. Use this strategy: after restricting, swap variables and solve, ensuring the branch aligns with the restriction— for $x \ge 0$, it's the positive root. This restriction lets you pick one branch, turning a non-invertible function into one with a clear inverse—keep practicing, you're doing awesome!