Connecting Infinite Limits and Vertical Asymptotes
Help Questions
AP Calculus AB › Connecting Infinite Limits and Vertical Asymptotes
For $f(x)=\dfrac{3}{x-2}$, which limit notation represents the behavior of $f(x)$ as $x$ approaches $2$ from the right?
$\displaystyle \lim_{x=2^+} f(x)=+\infty$
$\displaystyle f(2)=+\infty$
$\displaystyle \lim_{x\to2} f(x)=0$
$\displaystyle \lim_{x\to2^-} f(x)=+\infty$
$\displaystyle \lim_{x\to2^+} f(x)=+\infty$
Explanation
For f(x) = 3/(x-2), we need to identify the limit notation for the behavior as x approaches 2 from the right. The notation x→2⁺ means x approaches 2 from values greater than 2. As x gets closer to 2 from the right, the denominator (x-2) becomes a small positive number, making the fraction 3/(small positive) = large positive value. Therefore, lim[x→2⁺] f(x) = +∞ correctly represents this behavior. A common error is using an equals sign instead of an arrow (x=2⁺), which is incorrect notation. Another mistake is evaluating f(2), which is undefined since division by zero is not allowed. Notation checklist: Use → not =, include superscript + or - for one-sided limits, and remember that limits describe approach behavior, not function values.
Let $u(x)=\dfrac{-1}{(x-1)^2}$. Which limit notation represents the behavior as $x\to 1$?
$\displaystyle u(1)=-\infty$
$\displaystyle \lim_{x\to 1^+} u(x)=+\infty$
$\displaystyle \lim_{x\to 1^-} u(x)=+\infty$
$\displaystyle \lim_{x\to 1} u(x)=+\infty$
$\displaystyle \lim_{x\to 1} u(x)=-\infty$
Explanation
The function u(x) = $-1/(x-1)^2$ has an even-powered denominator, always positive near x=1, with negative numerator leading to -∞ from both sides. As x approaches 1, $(x-1)^2$ positive small, u(x) = -1/(positive) = large negative. The two-sided notation $lim_{x→1}$ u(x) = -∞ is correct since sides match. This is valid for symmetric negative infinite behavior. A common symbolic error is predicting +∞ by overlooking the negative sign, or writing u(1) = -∞, but limits aren't point evaluations. Ignoring the even power's positivity is frequent. Transferable notation checklist: Identify if the limit is one-sided or two-sided; determine the sign of the expression near the asymptote; use ±∞ appropriately; avoid equating to function value; specify direction with ^+ or ^- when necessary.
Let $g(x)=\dfrac{-4}{x+6}$. Which limit notation represents the behavior as $x\to -6^-$?
$\displaystyle \lim_{x\to -6} g(x)=0$
$\displaystyle \lim_{x\to -6^+} g(x)=-\infty$
$\displaystyle \lim_{x\to -6^-} g(x)=+\infty$
$\displaystyle \lim_{x\to -6^-} g(x)=-\infty$
$\displaystyle g(-6)=-\infty$
Explanation
The coefficient's sign affects the infinity direction in rational functions. For $g(x) = \dfrac{-4}{x+6}$, as $x \to -6^-$, $x+6$ negative small, $-4$/negative = positive large, so $\lim_{x\to -6^-} g(x) = +\infty$ correctly represents this. This notation is valid due to the negative numerator over negative denominator yielding positive. A common error is ignoring the numerator's sign, leading to incorrect $-\infty$. Using two-sided limit here would be wrong since sides differ. To ensure correct notation, use this transferable notation checklist: 1. Use 'lim' to indicate a limit, not the function equals notation. 2. Specify the direction with ^+ or ^- for one-sided limits. 3. Determine the sign ($+\infty$ or $-\infty$) based on whether the function increases or decreases without bound. 4. Remember that vertical asymptotes correspond to limits to infinity, and the function is undefined at that point.
For $g(x)=\dfrac{x^2}{x^2-1}$, which limit notation represents the behavior as $x\to 1^-$?
$\displaystyle \lim_{x\to 1^+} g(x)=-\infty$
$\displaystyle \lim_{x\to 1^-} g(x)=+\infty$
$\displaystyle \lim_{x\to 1^-} g(x)=-\infty$
$\displaystyle g(1)=-\infty$
$\displaystyle \lim_{x\to -1^-} g(x)=-\infty$
Explanation
Infinite limits are denoted when a function approaches positive or negative infinity near a point, often indicating a vertical asymptote. For g(x) = x²/(x²-1), as x approaches 1 from the left, the denominator approaches 0 from the negative side while the numerator is positive, resulting in g(x) approaching $-\infty$, so $\lim_{x\to 1^-} g(x) = -\infty$ is correct. This representation is valid because near x=1^-, (x-1) is negative and (x+1) positive, making the denominator negative, and positive over negative yields negative values growing in magnitude. A common symbolic error is writing g(1) = -∞, but limits do not assign a value to the function at the point. Confusing left and right directions is another frequent mistake, which changes the sign analysis. To ensure correct notation, use this transferable notation checklist: 1. Use 'lim' to indicate a limit, not the function equals notation. 2. Specify the direction with ^+ or ^- for one-sided limits. 3. Determine the sign ($+\infty$ or $-\infty$) based on whether the function increases or decreases without bound. 4. Remember that vertical asymptotes correspond to limits to infinity, and the function is undefined at that point.
For $g(x)=\dfrac{3}{x+1}$, which limit expression matches the behavior as $x\to -1^-$?
$\displaystyle \lim_{x\to -1^-} g(x)=-\infty$
$\displaystyle g(-1)= -\infty$
$\displaystyle \lim_{x\to -1^+} g(x)=-\infty$
$\displaystyle \lim_{x\to -1} g(x)=3$
$\displaystyle \lim_{x\to -1^-} g(x)=+\infty$
Explanation
For $g(x) = \dfrac{3}{x+1}$, the vertical asymptote is at $x=-1$ where the denominator is zero. As $x$ approaches -1 from the left ($x < -1$), $x+1$ is negative and approaching zero, so $g(x) = \dfrac{3}{\text{small negative}}$ becomes a large negative number, hence $-\infty$. The correct notation is $\lim_{x \to -1^-} g(x) = -\infty$, emphasizing the left-hand approach and negative infinity. This representation is valid as it describes the unbounded decrease without evaluating at $x=-1$. A common error is using two-sided limit notation when sides differ, or mistakenly writing $g(-1) = -\infty$, but limits describe approach, not point values. Students often mix up signs by not carefully checking the denominator's sign from each side. Transferable notation checklist: Identify if the limit is one-sided or two-sided; determine the sign of the expression near the asymptote; use $\pm \infty$ appropriately; avoid equating to function value; specify direction with $^{+}$ or $^{-}$ when necessary.
Let $g(x)=\dfrac{-4}{x+6}$. Which limit notation represents the behavior as $x\to -6^+$?
$\displaystyle \lim_{x\to -6^+} g(x)=-\infty$
$\displaystyle \lim_{x\to -6^-} g(x)=+\infty$
$\displaystyle \lim_{x\to -6} g(x)=-4$
$\displaystyle \lim_{x\to -6^+} g(x)=+\infty$
$\displaystyle g(-6)=+\infty$
Explanation
Direction matters in determining the sign for limits near asymptotes. For g(x) = -4/(x+6), as $x \to -6^+$, $x+6$ positive small, $-4/\text{positive} = \text{negative large}$, so $\lim_{x\to -6^+} g(x) = -\infty$ is the correct notation. This is valid because the negative numerator over positive denominator results in large negative values. A common symbolic error is misplacing the direction superscript, changing the behavior described. Writing $g(-6) = -\infty$ confuses limit with function value. To ensure correct notation, use this transferable notation checklist: 1. Use 'lim' to indicate a limit, not the function equals notation. 2. Specify the direction with ^+ or ^- for one-sided limits. 3. Determine the sign (+∞ or -∞) based on whether the function increases or decreases without bound. 4. Remember that vertical asymptotes correspond to limits to infinity, and the function is undefined at that point.
Let $f(x)=\dfrac{2}{x(x-4)}$. Which limit notation matches the behavior as $x\to 4^-$?
$\displaystyle \lim_{x\to 4^-} f(x)=+\infty$
$\displaystyle \lim_{x\to 4} f(x)=0$
$\displaystyle \lim_{x\to 4^+} f(x)=-\infty$
$\displaystyle f(4)=-\infty$
$\displaystyle \lim_{x\to 4^-} f(x)=-\infty$
Explanation
For f(x) = 2/(x(x-4)), as x→4^-, x positive ≈4, (x-4) negative small, denominator negative small, 2/negative = -∞. The notation $lim_{x→4^-}$ f(x) = -∞ fits. This is valid based on left-hand signs. A frequent mistake is using +∞ or equating to f(4) = -∞. Failing to check the (x-4) negativity is common. Transferable notation checklist: Identify if the limit is one-sided or two-sided; determine the sign of the expression near the asymptote; use ±∞ appropriately; avoid equating to function value; specify direction with ^+ or ^- when necessary.
For $f(x)=\dfrac{1}{x-8}$, which limit notation represents the behavior as $x\to 8^-$?
$\displaystyle \lim_{x\to 8^+} f(x)=-\infty$
$\displaystyle \lim_{x\to 8^-} f(x)=-\infty$
$\displaystyle f(8)=-\infty$
$\displaystyle \lim_{x\to 8^-} f(x)=+\infty$
$\displaystyle \lim_{x\to 8} f(x)=8$
Explanation
For rational functions, sign analysis near poles determines the infinity direction. For f(x) = 1/(x-8), as x → 8^-, the denominator is negative small, so f(x) → -∞, and \(\lim_{x\to 8^-} f(x) = -\infty\) is correct. This representation is valid based on the sign of the denominator approaching from the left. A common error is writing the two-sided limit when sides differ. Using f(8) = -∞ is symbolically wrong, as it's not a function value. To ensure correct notation, use this transferable notation checklist: 1. Use 'lim' to indicate a limit, not the function equals notation. 2. Specify the direction with ^+ or ^- for one-sided limits. 3. Determine the sign (+∞ or -∞) based on whether the function increases or decreases without bound. 4. Remember that vertical asymptotes correspond to limits to infinity, and the function is undefined at that point.
For $p(x)=\dfrac{1}{(x-3)^2}$, which limit expression correctly describes the behavior as $x\to 3$?
$\displaystyle \lim_{x\to 3^-} p(x)=-\infty$
$\displaystyle \lim_{x\to 3^+} p(x)=-\infty$
$\displaystyle p(3)=+\infty$
$\displaystyle \lim_{x\to 3} p(x)=+\infty$
$\displaystyle \lim_{x\to 3} p(x)=0$
Explanation
For p(x) = $1/(x-3)^2$, the denominator is always positive near x=3 except at the point itself, leading to +∞ from both sides. As x approaches 3 from either side, $(x-3)^2$ is positive small, so p(x) becomes large positive. The two-sided limit notation $lim_{x→3}$ p(x) = +∞ is appropriate since both one-sided limits agree. This is valid for even-powered denominators causing symmetric behavior to +∞. A common error is writing one-sided notation unnecessarily or using p(3) = +∞, but limits aren't function values. Students sometimes incorrectly predict -∞ by ignoring the squaring effect. Transferable notation checklist: Identify if the limit is one-sided or two-sided; determine the sign of the expression near the asymptote; use ±∞ appropriately; avoid equating to function value; specify direction with ^+ or ^- when necessary.
For $f(x)=\dfrac{2}{(x+5)^2}$, which limit notation describes the behavior as $x\to -5$?
$\displaystyle f(-5)=+\infty$
$\displaystyle \lim_{x\to -5^+} f(x)=-\infty$
$\displaystyle \lim_{x\to -5^-} f(x)=-\infty$
$\displaystyle \lim_{x\to -5} f(x)=+\infty$
$\displaystyle \lim_{x\to -5} f(x)=2$
Explanation
For f(x) = $2/(x+5)^2$, the even power ensures positive denominator near x=-5 from both sides, leading to +∞. The two-sided limit $lim_{x→-5}$ f(x) = +∞ is correct. This is valid for symmetric positive behavior. A common error is predicting differing sides or using f(-5) = +∞. Overlooking the square's effect is typical. Transferable notation checklist: Identify if the limit is one-sided or two-sided; determine the sign of the expression near the asymptote; use ±∞ appropriately; avoid equating to function value; specify direction with ^+ or ^- when necessary.