Rational Functions and Holes
Help Questions
AP Precalculus › Rational Functions and Holes
Consider the function $g(x)=\dfrac{x^2-4}{x-2}$. At which $x$-value is $g$ undefined due to a removable discontinuity?
$x=2$
$x=4$
$x=0$
$x=-2$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole (removable discontinuity) occurs where both numerator and denominator share a common factor that equals zero. In this problem, g(x) = (x²-4)/(x-2) can be factored as g(x) = (x+2)(x-2)/(x-2), showing the common factor (x-2). Choice B is correct because at x = 2, both the numerator (2²-4 = 0) and denominator (2-2 = 0) equal zero, creating the removable discontinuity. Choice A is incorrect as it confuses the zero of the simplified function with the hole's location. To help students: Use the difference of squares factoring pattern. Remind them that after canceling, the hole remains at the canceled factor's zero.
Consider the function $u(x)=\dfrac{x^2-25}{x-5}$ for a fuel-usage model; what is the location of the hole in the function $u(x)$?
$x=1$
$x=5$
$x=-5$
$x=0$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function u(x) = (x²-25)/(x-5) can be factored as u(x) = (x+5)(x-5)/(x-5), using the difference of squares pattern, revealing a common factor of (x-5). Choice B is correct because when x = 5, both the numerator (25-25=0) and denominator (5-5=0) equal zero, and the (x-5) factor cancels out, creating a hole at x = 5. Choice A is incorrect because x = -5 only makes the numerator zero after cancellation, creating a zero of the simplified function. To help students: Remember that x²-a² = (x+a)(x-a) for difference of squares. Use algebraic cancellation to identify removable discontinuities versus non-removable ones.
Consider the function $u(x)=\dfrac{x^2-1}{x+1}$ for a simplified index; at which $x$-value is it undefined due to a removable discontinuity?
$x=2$
$x=-1$
$x=0$
$x=1$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function u(x) = (x²-1)/(x+1) can be factored as u(x) = (x+1)(x-1)/(x+1), showing a common factor of (x+1). Choice B is correct because when x = -1, both the numerator (1-1=0) and denominator (-1+1=0) equal zero, and the (x+1) factor cancels, creating a hole at x = -1. Choice A is incorrect because x = 1 only makes the numerator zero after factoring, not the denominator. To help students: Recognize x²-1 as a difference of squares. Pay attention to the sign in the denominator (x+1) to identify the correct hole location.
Consider the function $g(x)=\dfrac{x^2-4x}{x-4}$ for a pricing rule; which value of $x$ makes the numerator and denominator zero, creating a hole?
$x=0$
$x=-4$
$x=2$
$x=4$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function g(x) = (x²-4x)/(x-4) can be factored as g(x) = x(x-4)/(x-4), showing a common factor of (x-4). Choice A is correct because when x = 4, both the numerator (16-16=0) and denominator (4-4=0) equal zero, and the (x-4) factor cancels out, creating a hole at x = 4. Choice B is incorrect because x = 0 only makes the numerator zero, resulting in a zero of the function rather than a hole. To help students: Factor the numerator completely before identifying common factors. Use substitution to verify that both numerator and denominator equal zero at the hole location.
Consider the function $h(x)=\dfrac{x^2-1}{x-1}$ for a sensor calibration; what is the location of the hole in the function $h(x)$?
$x=0$
$x=1$
$x=-1$
$x=2$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function h(x) = (x²-1)/(x-1) can be factored as h(x) = (x+1)(x-1)/(x-1), revealing a common factor of (x-1). Choice B is correct because when x = 1, both the numerator (1-1=0) and denominator (1-1=0) equal zero, and the (x-1) factor cancels out, creating a hole at x = 1. Choice A is incorrect because x = -1 only makes the numerator zero, which creates a zero of the simplified function, not a hole. To help students: Remember the difference of squares factoring pattern (a²-b² = (a+b)(a-b)). Practice identifying and canceling common factors to find removable discontinuities.
Consider the function $q(x)=\dfrac{x^2-2x-3}{x-3}$ for a water-flow model; which value of $x$ makes the numerator and denominator zero, creating a hole?
$x=-3$
$x=3$
$x=1$
$x=-1$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function q(x) = (x²-2x-3)/(x-3) can be factored as q(x) = (x-3)(x+1)/(x-3), revealing a common factor of (x-3). Choice B is correct because when x = 3, both the numerator (9-6-3=0) and denominator (3-3=0) equal zero, and the (x-3) factor cancels out, creating a hole at x = 3. Choice D is incorrect because x = -3 doesn't make either the numerator or denominator zero. To help students: Use factoring techniques for trinomials where the leading coefficient is 1. Check your factorization by expanding to verify it matches the original expression.
Consider the function $v(x)=\dfrac{x^2+3x}{x}$ for a unit-rate calculation; at which $x$-value is $v$ undefined due to a removable discontinuity?
$x=-3$
$x=1$
$x=0$
$x=3$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function v(x) = (x²+3x)/x can be factored as v(x) = x(x+3)/x, showing a common factor of x. Choice C is correct because when x = 0, both the numerator (0+0=0) and denominator (0) equal zero, and the x factor cancels out, creating a hole at x = 0. Choice A is incorrect because x = -3 only makes the numerator zero after cancellation, not creating a hole. To help students: Factor out common terms from the numerator before identifying cancelable factors. Be careful with x = 0 cases, as they often create holes when x appears in both numerator and denominator.
Consider $h(x)=\dfrac{x^2+5x+6}{x+2}$. Which value of $x$ makes numerator and denominator zero, creating a hole?
$x=-3$
$x=5$
$x=-2$
$x=2$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, h(x) = (x²+5x+6)/(x+2) requires factoring the numerator: x²+5x+6 = (x+2)(x+3), revealing the common factor (x+2). Choice A is correct because when x = -2, both the numerator (4-10+6 = 0) and denominator (-2+2 = 0) equal zero, confirming the hole's location. Choice B is incorrect as it identifies where the simplified function h(x) = x+3 equals zero. To help students: Practice factoring quadratic expressions. Emphasize checking that both original numerator and denominator equal zero at the proposed hole location.
Consider the function $p(x)=\dfrac{x^2+x}{x}$ for a per-item cost model; at which $x$-value is it undefined due to a removable discontinuity?
$x=1$
$x=0$
$x=2$
$x=-1$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function p(x) = (x²+x)/x can be factored as p(x) = x(x+1)/x, showing a common factor of x. Choice C is correct because when x = 0, both the numerator (0+0=0) and denominator (0) equal zero, and the x factor cancels, creating a hole at x = 0. Choice A is incorrect because x = 1 doesn't make the denominator zero. To help students: Factor out common terms from the numerator before simplifying. Remind students that division by zero creates discontinuities, but when both parts are zero, it's removable.
Consider the function $q(x)=\dfrac{x^2-6x+9}{x-3}$ for a scaling rule; what is the location of the hole?
$x=3$
$x=9$
$x=6$
$x=-3$
Explanation
This question tests AP Precalculus skills: understanding of rational functions and identifying holes. A hole occurs where a rational function's numerator and denominator both equal zero, creating a removable discontinuity. In this problem, the function q(x) = (x²-6x+9)/(x-3) has a numerator that factors as (x-3)², so q(x) = (x-3)²/(x-3), with a common factor of (x-3). Choice B is correct because when x = 3, both the numerator (9-18+9=0) and denominator (3-3=0) equal zero, and one (x-3) factor cancels, creating a hole at x = 3. Choice A is incorrect because x = -3 doesn't make either part zero. To help students: Recognize perfect square trinomials like x²-6x+9 = (x-3)². Practice factoring quadratics to identify common factors with the denominator.