Rational Functions and Vertical Asymptotes

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AP Precalculus › Rational Functions and Vertical Asymptotes

Questions 1 - 10
1

Which of the following rational functions has a vertical asymptote at $$x = 3$$?

$$f(x) = \frac{x + 2}{x - 3}$$

$$f(x) = \frac{x^2 - 9}{x - 3}$$

$$f(x) = \frac{x - 3}{x + 1}$$

$$f(x) = \frac{x^2 - 6x + 9}{x^2 - 4}$$

Explanation

A vertical asymptote occurs where the denominator equals zero but the numerator does not. For choice B, when $$x = 3$$, the denominator $$x - 3 = 0$$ and the numerator $$x + 2 = 5 \neq 0$$, creating a vertical asymptote. Choice A has a hole at $$x = 3$$ since both numerator and denominator equal zero. Choice C has vertical asymptotes at $$x = \pm 2$$, not $$x = 3$$. Choice D has a vertical asymptote at $$x = -1$$, not $$x = 3$$.

2

The rational function $$g(x) = \frac{2x - 5}{(x + 1)(x - 4)}$$ has vertical asymptotes at which values of $$x$$?

$$x = -1$$ and $$x = \frac{5}{2}$$ only

$$x = -1$$ and $$x = 4$$ only

$$x = \frac{5}{2}$$ and $$x = 4$$ only

$$x = 1$$ and $$x = -4$$ only

Explanation

Vertical asymptotes occur where the denominator equals zero but the numerator does not. The denominator $$(x + 1)(x - 4) = 0$$ when $$x = -1$$ or $$x = 4$$. At $$x = -1$$: numerator $$= 2(-1) - 5 = -7 \neq 0$$. At $$x = 4$$: numerator $$= 2(4) - 5 = 3 \neq 0$$. Both create vertical asymptotes. Choice B incorrectly includes $$x = \frac{5}{2}$$ (where numerator equals zero). Choice C incorrectly includes $$x = \frac{5}{2}$$. Choice D uses incorrect signs.

3

For what value(s) of $$k$$ does the rational function $$f(x) = \frac{x + k}{x^2 - 5x + 6}$$ have exactly two vertical asymptotes?

$$k$$ must equal $$2$$ or $$3$$ for two vertical asymptotes

$$k$$ can be any real number except $$k = -2$$ and $$k = -3$$

$$k$$ can be any real number except $$k = 2$$ and $$k = 3$$

$$k$$ can be any real number except $$k = -6$$ and $$k = 1$$

Explanation

The denominator factors as $$x^2 - 5x + 6 = (x - 2)(x - 3)$$, giving potential vertical asymptotes at $$x = 2$$ and $$x = 3$$. For exactly two vertical asymptotes, the numerator must not equal zero at either $$x = 2$$ or $$x = 3$$. At $$x = 2$$: numerator $$= 2 + k$$, so $$k \neq -2$$. At $$x = 3$$: numerator $$= 3 + k$$, so $$k \neq -3$$. Therefore $$k$$ can be any real number except $$-2$$ and $$-3$$. Choice B uses wrong signs. Choice C would create holes, not asymptotes. Choice D uses incorrect values.

4

The function $$h(x) = \frac{2x + 3}{x^2 - 9}$$ has vertical asymptotes that can be described by which limit statements?

$$\lim_{x \to 3^-} h(x) = +\infty$$ and $$\lim_{x \to -3^+} h(x) = +\infty$$

$$\lim_{x \to 3^-} h(x) = -\infty$$ and $$\lim_{x \to -3^+} h(x) = -\infty$$

$$\lim_{x \to 3^+} h(x) = -\infty$$ and $$\lim_{x \to -3^-} h(x) = +\infty$$

$$\lim_{x \to 3^-} h(x) = +\infty$$ and $$\lim_{x \to -3^-} h(x) = -\infty$$

Explanation

The denominator $$x^2 - 9 = (x - 3)(x + 3)$$ gives vertical asymptotes at $$x = 3$$ and $$x = -3$$. As $$x \to 3^-$$: numerator approaches $$9 > 0$$ and denominator $$(x-3)(x+3)$$ approaches $$0^- \cdot 6 = 0^-$$, so the limit is $$+\infty$$. As $$x \to -3^-$$: numerator approaches $$-3 < 0$$ and denominator $$(-6)(x+3)$$ approaches $$(-6)(0^-) = 0^+$$, so the limit is $$-\infty$$. Choices B, C, and D have incorrect sign combinations for the limit behavior.

5

For the rational function $$f(x) = \frac{5x - 1}{(x + 3)^2(x - 7)}$$, which statement about the multiplicity of vertical asymptotes is correct?

There is a vertical asymptote of multiplicity 3 at $$x = -3$$ only

There is a vertical asymptote of multiplicity 1 at $$x = -3$$ and multiplicity 2 at $$x = 7$$

There is a vertical asymptote of multiplicity 2 at $$x = -3$$ and multiplicity 1 at $$x = 7$$

There are vertical asymptotes of multiplicity 1 at both $$x = -3$$ and $$x = 7$$

Explanation

The denominator $$(x + 3)^2(x - 7)$$ has zeros at $$x = -3$$ (multiplicity 2) and $$x = 7$$ (multiplicity 1). Since the numerator $$5x - 1$$ doesn't equal zero at either point ($$5(-3) - 1 = -16 \neq 0$$ and $$5(7) - 1 = 34 \neq 0$$), both zeros create vertical asymptotes with their respective multiplicities. The asymptote at $$x = -3$$ has multiplicity 2, and at $$x = 7$$ has multiplicity 1. Choices B and C have incorrect multiplicities. Choice D ignores the asymptote at $$x = 7$$.

6

Which rational function has exactly one vertical asymptote at $$x = 4$$?

$$f(x) = \frac{x + 1}{x - 4}$$

$$f(x) = \frac{x^2 + 1}{(x - 4)(x + 1)}$$

$$f(x) = \frac{2x + 3}{x^2 - 8x + 16}$$

$$f(x) = \frac{x - 4}{x^2 - 16}$$

Explanation

For exactly one vertical asymptote at $$x = 4$$, the denominator must have $$x = 4$$ as its only zero where the numerator is non-zero. Choice B has denominator $$x - 4$$ with only one zero at $$x = 4$$, and numerator $$4 + 1 = 5 \neq 0$$ at this point. Choice A has two asymptotes at $$x = 4$$ and $$x = -1$$. Choice C has $$x^2 - 16 = (x-4)(x+4)$$, giving asymptotes at both $$x = 4$$ and $$x = -4$$. Choice D has $$x^2 - 8x + 16 = (x-4)^2$$, giving one asymptote at $$x = 4$$ with multiplicity 2.

7

Which rational function has vertical asymptotes at $$x = 1$$ and $$x = -5$$, but no other vertical asymptotes?

$$f(x) = \frac{2x - 3}{x^2 + 4x - 5}$$

$$f(x) = \frac{x^2 + 1}{(x + 1)(x - 5)}$$

$$f(x) = \frac{x + 2}{(x - 1)(x + 5)}$$

$$f(x) = \frac{3x + 7}{x^2 - 6x + 5}$$

Explanation

For vertical asymptotes at $$x = 1$$ and $$x = -5$$, the denominator must have these as zeros where the numerator is non-zero. Choice B has denominator $$x^2 + 4x - 5 = (x - 1)(x + 5)$$, giving zeros at $$x = 1$$ and $$x = -5$$. Check numerator: at $$x = 1$$, $$2(1) - 3 = -1 \neq 0$$; at $$x = -5$$, $$2(-5) - 3 = -13 \neq 0$$. Both create asymptotes. Choice A has wrong signs. Choice C has asymptotes at $$x = -1$$ and $$x = 5$$. Choice D has $$x^2 - 6x + 5 = (x-1)(x-5)$$, giving asymptotes at $$x = 1$$ and $$x = 5$$.

8

The rational function $$h(x) = \frac{2x + 1}{x^3 - 2x^2 - 3x}$$ has vertical asymptotes at which values?

$$x = 0$$ and $$x = 3$$ are vertical asymptotes, but $$x = -1$$ is not

$$x = 0$$ is the only vertical asymptote of the function

$$x = 0$$, $$x = 3$$, and $$x = -1$$ are all vertical asymptotes

$$x = 3$$ and $$x = -1$$ are vertical asymptotes, but $$x = 0$$ is not

Explanation

Factor the denominator: $$x^3 - 2x^2 - 3x = x(x^2 - 2x - 3) = x(x - 3)(x + 1)$$. The denominator has zeros at $$x = 0, 3, -1$$. Check if the numerator $$2x + 1$$ equals zero at any of these points: At $$x = 0$$: $$2(0) + 1 = 1 \neq 0$$. At $$x = 3$$: $$2(3) + 1 = 7 \neq 0$$. At $$x = -1$$: $$2(-1) + 1 = -1 \neq 0$$. Since the numerator is non-zero at all three zeros of the denominator, all three create vertical asymptotes. Choices B, C, and D incorrectly exclude some asymptotes.

9

The rational function $$h(x) = \frac{x^2 + 5x + 6}{x^2 + 3x + 2}$$ has how many vertical asymptotes?

The function has exactly one vertical asymptote

The function has exactly three vertical asymptotes

The function has exactly two vertical asymptotes

The function has no vertical asymptotes

Explanation

Factor both parts: numerator $$x^2 + 5x + 6 = (x + 2)(x + 3)$$ and denominator $$x^2 + 3x + 2 = (x + 1)(x + 2)$$. So $$h(x) = \frac{(x + 2)(x + 3)}{(x + 1)(x + 2)} = \frac{x + 3}{x + 1}$$ for $$x \neq -2$$. After canceling the common factor $$(x + 2)$$, there's a hole at $$x = -2$$ and one vertical asymptote at $$x = -1$$ where the simplified denominator equals zero but the numerator $$(-1) + 3 = 2 \neq 0$$. Choice A counts the hole as an asymptote. Choices C and D are incorrect counts.

10

Which rational function has a vertical asymptote at $$x = -2$$ but no vertical asymptote at $$x = 3$$?

$$f(x) = \frac{x^2 - 9}{x^2 - x - 6}$$

$$f(x) = \frac{x + 2}{x - 3}$$

$$f(x) = \frac{x - 3}{(x + 2)(x - 3)}$$

$$f(x) = \frac{2x + 1}{x + 2}$$

Explanation

For a vertical asymptote at $$x = -2$$, the denominator must equal zero at $$x = -2$$ while the numerator does not. Choice C has denominator $$x + 2$$ which equals zero when $$x = -2$$, and numerator $$2(-2) + 1 = -3 \neq 0$$, creating a vertical asymptote. Choice A has a hole at $$x = 3$$ and asymptote at $$x = -2$$. Choice B has asymptote at $$x = 3$$, not $$x = -2$$. Choice D factors to $$\frac{(x-3)(x+3)}{(x-3)(x+2)}$$ with asymptote at $$x = -2$$ after canceling.

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