Connecting Infinite Limits and Vertical Asymptotes
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AP Calculus BC › Connecting Infinite Limits and Vertical Asymptotes
If $q(x)=\dfrac{2}{x-4}+1$, which statement correctly identifies the vertical asymptote using limits?
$\lim_{x\to4} q(x)=0$, so there is a removable discontinuity at $x=4$
$\lim_{x\to4^-} q(x)=-\infty$ and $\lim_{x\to4^+} q(x)=+\infty$, so $x=4$ is a vertical asymptote
$\lim_{x\to4^-} q(x)=+\infty$ and $\lim_{x\to4^+} q(x)=-\infty$, so $y=4$ is a vertical asymptote
$\lim_{x\to\infty} q(x)=1$, so $x=1$ is a vertical asymptote
$\lim_{x\to4} q(x)=1$, so $y=1$ is a vertical asymptote
Explanation
This problem tests linking infinite limits to vertical asymptotes in functions with added constants. For q(x) = 2/(x-4) + 1, the denominator zero at x=4 with nonzero numerator term drives the behavior. Approaching from the left yields negative infinity, and from the right positive infinity, despite the +1 not affecting the infinite trend. These opposing infinite limits confirm a vertical asymptote at x=4. A common distractor confuses this with a horizontal asymptote at y=1 due to the added constant, but horizontal asymptotes relate to limits at infinity, not vertical ones. A general strategy is to isolate terms causing division by zero and evaluate one-sided limits for infinity.
For $u(x)=\dfrac{5}{x}-2$, which statement correctly describes the vertical asymptote and limit at it?
No vertical asymptote because subtracting 2 shifts the graph down.
Vertical asymptote at $y=0$ and $\lim_{x\to0} u(x)=-2$.
Horizontal asymptote at $x=0$ and $\lim_{x\to0} u(x)=\infty$.
Vertical asymptote at $x=-2$ and $\lim_{x\to-2} u(x)=\infty$.
Vertical asymptote at $x=0$ and $\lim_{x\to0^+} u(x)=\infty$, $\lim_{x\to0^-} u(x)=-\infty$.
Explanation
This question tests your ability to connect infinite limits with vertical asymptotes in transformed reciprocal functions. For u(x) = 5/x - 2, the 5/x term causes infinite behavior at x=0: positive infinity from the right and negative infinity from the left. Subtracting 2 shifts the graph vertically but doesn't prevent the limits from being infinite. Thus, there's a vertical asymptote at x=0 with opposite-signed infinite limits. Choice D is tempting but fails because the shift affects horizontal asymptotes, not the vertical one from the 1/x core. Always isolate the term causing division by zero and assess its infinite limits, regardless of added constants.
For $q(x)=\dfrac{2}{x-4}+1$, which statement correctly describes the vertical asymptote?
$\lim_{x\to \infty} q(x)=\infty$.
$\lim_{x\to 4} q(x)=1$.
There is a vertical asymptote at $x=4$.
There is a vertical asymptote at $x=1$.
There is a horizontal asymptote at $x=4$.
Explanation
This question tests your ability to identify vertical asymptotes in transformed rational functions. For $q(x)=\frac{2}{x-4}+1$, the rational part $\frac{2}{x-4}$ has a denominator that equals zero when $x=4$. As $x\to 4^-$, we have $(x-4)\to 0^-$, so $\frac{2}{x-4}\to -\infty$, making $q(x)\to -\infty$. As $x\to 4^+$, we have $(x-4)\to 0^+$, so $\frac{2}{x-4}\to +\infty$, making $q(x)\to +\infty$. This confirms a vertical asymptote at $x=4$. Choice D incorrectly calls this a horizontal asymptote, confusing the direction of the asymptote line. Remember that vertical asymptotes are vertical lines $x=a$ where the function has infinite limits, while horizontal asymptotes are horizontal lines $y=b$ describing end behavior.
For $f(x)=\dfrac{3x+1}{x^2-4}$, which statement correctly describes the vertical asymptote behavior?
The graph has a horizontal asymptote $y=\dfrac{3}{2}$.
There is a vertical asymptote at $x=0$.
$\lim_{x\to 2} f(x)$ exists and is finite.
$\lim_{x\to -2} f(x)=0$.
There are vertical asymptotes at $x=-2$ and $x=2$.
Explanation
This question tests your ability to identify vertical asymptotes by analyzing when denominators equal zero and limits become infinite. For $f(x)=\frac{3x+1}{x^2-4}$, we need to find where the denominator $x^2-4=0$, which factors as $(x-2)(x+2)=0$, giving us $x=2$ and $x=-2$. At both these values, the numerator $3x+1$ is non-zero (it equals 7 and -5 respectively), so the function has infinite limits at these points. Since the denominator changes sign as we cross each zero, we get vertical asymptotes at both $x=-2$ and $x=2$. Choice A incorrectly suggests an asymptote at $x=0$ where the function is actually well-defined. When finding vertical asymptotes, always check that the numerator doesn't also equal zero at the same point, as that could indicate a removable discontinuity instead.
For $u(x)=\dfrac{x-1}{\sqrt{x-1}}$ with domain $x>1$, which statement describes $\lim_{x\to1^+} u(x)$?
$\lim_{x\to1^+} u(x)=0$, so $x=1$ is not a vertical asymptote
$\lim_{x\to1^+} u(x)$ does not exist, so $y=0$ is a vertical asymptote
$\lim_{x\to1^+} u(x)=1$, so $y=1$ is a vertical asymptote
$\lim_{x\to1^+} u(x)=-\infty$, so $x=1$ is a vertical asymptote
$\lim_{x\to1^+} u(x)=\infty$, so $x=1$ is a vertical asymptote
Explanation
This problem tests simplifying algebraic expressions before taking limits. For $u(x)=\frac{x-1}{\sqrt{x-1}}$ with $x>1$, we can rewrite this as $u(x)=\frac{x-1}{(x-1)^{1/2}}=(x-1)^{1-1/2}=(x-1)^{1/2}=\sqrt{x-1}$. As $x\to1^+$, we have $\sqrt{x-1}\to\sqrt{0^+}=0$. Since the limit is finite (zero), there is no vertical asymptote at $x=1$. Choice A incorrectly assumes an infinite limit without simplifying. Always simplify expressions algebraically before evaluating limits to avoid missing cancellations.
Consider $t(x)=\dfrac{2}{x-7}+5$. Which statement correctly describes the vertical asymptote and associated infinite limit?
Vertical asymptote at $x=5$ with $\lim_{x\to 5^+}t(x)=+\infty$
Vertical asymptote at $x=7$ with $\lim_{x\to 7^+}t(x)=+\infty$
Horizontal asymptote at $x=7$ with $\lim_{x\to 7^+}t(x)=+\infty$
Vertical asymptote at $y=7$ with $\lim_{x\to 7^+}t(x)=+\infty$
Finite limit $\lim_{x\to 7}t(x)=5$ and no vertical asymptote
Explanation
This problem requires recognizing vertical asymptotes in transformed rational functions through infinite limit behavior. The function t(x) = 2/(x-7) + 5 is a transformation of the basic rational function 1/x. The term 2/(x-7) has a vertical asymptote at x = 7, where the denominator equals zero. As x approaches 7 from the right (x → 7⁺), (x-7) approaches 0 through positive values, making 2/(x-7) approach +∞, and thus t(x) approaches +∞. The constant term +5 shifts the graph vertically but doesn't affect the location of the vertical asymptote. Choice C incorrectly places the asymptote at y = 7, confusing vertical and horizontal asymptotes. For functions of the form a/(x-h) + k, vertical asymptotes always occur at x = h.
For $f(x)=\dfrac{3x+1}{(x-2)(x+4)}$, which statement correctly describes the vertical asymptote behavior near $x=2$?
$\lim_{x\to2^-}f(x)=\infty$ and $\lim_{x\to2^+}f(x)=-\infty$
$\lim_{x\to2^-}f(x)=\infty$ and $\lim_{x\to2^+}f(x)=\infty$
$\lim_{x\to2}f(x)=\dfrac{7}{6}$
$\lim_{x\to2^-}f(x)=0$ and $\lim_{x\to2^+}f(x)=0$
$\lim_{x\to2^-}f(x)=-\infty$ and $\lim_{x\to2^+}f(x)=\infty$
Explanation
This question tests your ability to connect infinite limits with vertical asymptote behavior. For $f(x)=\frac{3x+1}{(x-2)(x+4)}$, as $x$ approaches 2, the denominator approaches 0 while the numerator approaches $3(2)+1=7$. When $x$ approaches 2 from the left ($x<2$), the factor $(x-2)$ is negative and $(x+4)$ is positive, making the denominator negative overall, so $f(x)\to-\infty$. When $x$ approaches 2 from the right ($x>2$), the factor $(x-2)$ is positive and $(x+4)$ is positive, making the denominator positive, so $f(x)\to+\infty$. Choice A incorrectly claims both one-sided limits approach $+\infty$, failing to account for the sign change. The key strategy is to analyze the sign of each factor near the asymptote to determine whether the function approaches $+\infty$ or $-\infty$ from each side.
For $r(x)=\dfrac{2}{x^2-4}$, which statement correctly describes the vertical asymptote behavior near $x=2$?
$\lim_{x\to2^-}r(x)=0$ and $\lim_{x\to2^+}r(x)=0$
$\lim_{x\to2^-}r(x)=\infty$ and $\lim_{x\to2^+}r(x)=-\infty$
$\lim_{x\to2^-}r(x)=\infty$ and $\lim_{x\to2^+}r(x)=\infty$
$\lim_{x\to2^-}r(x)=-\infty$ and $\lim_{x\to2^+}r(x)=\infty$
$\lim_{x\to2}r(x)=\dfrac{1}{2}$
Explanation
This problem requires analyzing infinite limits for a rational function with a factorable denominator. For $r(x)=\frac{2}{x^2-4}=\frac{2}{(x-2)(x+2)}$, near $x=2$, the factor $(x+2)$ is approximately 4 (positive). When approaching from the left ($x<2$), the factor $(x-2)$ is negative, making the denominator negative overall, so $r(x)\to-\infty$. When approaching from the right ($x>2$), the factor $(x-2)$ is positive, making the denominator positive, so $r(x)\to+\infty$. This matches choice C, confirming that $x=2$ is a vertical asymptote with different one-sided infinite limits. Choice E incorrectly claims both limits are $+\infty$, missing the sign change. The strategy is to determine the sign of each factor in the denominator on each side of the potential asymptote.
If $\lim_{x\to-3^-} v(x)=-\infty$ and $\lim_{x\to-3^+} v(x)=5$, what best describes $x=-3$?
$x=-3$ is a vertical asymptote only if both one-sided limits are infinite.
$x=-3$ is a vertical asymptote because at least one one-sided limit is infinite.
$x=-3$ is removable because one one-sided limit is finite.
$x=-3$ is neither an asymptote nor a discontinuity because limits disagree.
$x=-3$ is a horizontal asymptote because one limit equals 5.
Explanation
This question tests your ability to connect infinite limits with vertical asymptotes when one-sided limits differ. If the left-hand limit at x=-3 is negative infinity and the right-hand is finite (5), the infinite side indicates unbounded behavior approaching x=-3. This supports a vertical asymptote, even if only one side is infinite. The finite side means the graph approaches 5 from the right, but the infinite side dominates for asymptote classification. Choice E is tempting but fails because vertical asymptotes require at least one infinite one-sided limit, not necessarily both. Always evaluate both one-sided limits separately; an infinite result from either confirms a vertical asymptote.
For $f(x)=\dfrac{3x-1}{(x+2)(x-5)}$, which statement correctly describes the vertical asymptote behavior?
No vertical asymptotes because the degree of numerator is less
Horizontal asymptote at $x=-2$ and $x=5$
Finite limits at $x=-2$ and $x=5$
Vertical asymptote at $y=-2$ and $y=5$
Vertical asymptotes at $x=-2$ and $x=5$
Explanation
This question tests your ability to identify vertical asymptotes by analyzing infinite limits of rational functions. The function f(x) = (3x-1)/[(x+2)(x-5)] has its denominator equal to zero when x = -2 and x = 5, while the numerator is non-zero at these values. As x approaches -2 or 5, the denominator approaches 0 while the numerator approaches finite non-zero values, causing the function to approach ±∞. This creates vertical asymptotes at x = -2 and x = 5. Choice B incorrectly identifies these as horizontal asymptotes, which would be lines of the form y = k, not x = k. When a rational function's denominator has zeros that don't cancel with the numerator, vertical asymptotes occur at those x-values.