Solving the Slant Asymptote of $$\frac{{x}^{3}-5{x}^{2}+4x}{-4{x}^{2}+36}$$

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Discussion Overview

The discussion revolves around finding the slant asymptote of the rational function $$\frac{{x}^{3}-5{x}^{2}+4x}{-4{x}^{2}+36}$$. Participants explore methods such as polynomial long division and limits to determine the coefficients of the asymptote.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant suggests that the slant asymptote can be expressed as $$mx+b$$, proposing that $$m = -\frac{1}{4}$$ but is uncertain about the value of $$b$$.
  • Another participant recommends performing polynomial long division to find the asymptote, noting that the remainder will approach zero for large values of $$x$$.
  • A different approach is presented involving limits to isolate $$b$$, with a reminder that for some functions, limits as $$x \to -\infty$$ may also be necessary.
  • One participant expresses confusion about obtaining the correct remainder during long division, indicating a possible discrepancy in their calculations.
  • Another participant provides a detailed long division process, concluding that the asymptote is $$y = -\frac{1}{4}x + \frac{5}{4}$$ and notes that the remainder will diminish for large $$x$$.
  • A later reply confirms the asymptote found by the previous participant and describes a verification method for the long division result.
  • One participant expresses curiosity about the LaTeX formatting used for long division.

Areas of Agreement / Disagreement

There is a general agreement on the form of the slant asymptote as $$y = -\frac{1}{4}x + \frac{5}{4}$$, but uncertainty remains regarding the calculation of $$b$$ and the correctness of the long division process among some participants.

Contextual Notes

Participants have not fully resolved the discrepancies in long division results, and there are varying approaches to determining the value of $$b$$, indicating potential limitations in their methods.

karush
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$$\frac{{x}^{3}-5{x}^{2}+4x}{-4{x}^{2}+36 }$$

Has a slant asymtope of $mx+b$ of which I got $-\frac{1}{4}x+b$
I couldn't get the b
 
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What do you get if you perform polynomial long division? The quotient will be the asymptote, as the remainder will tend to zero for values of $x$ having great magnitude.
 
Alternatively,

$$y=mx+b$$

$$b=y-mx=\lim_{x\to\infty}\dfrac{x^3-5x^2+4x}{-4x^2+36}-\lim_{x\to\infty}-\dfrac x4$$

Simplify and evaluate the limit to find $b$. For other functions it may be necessary to evaluate $x\to-\infty$.
 
I divided but got $-5{x}^{2 }-13x$ for a remainder?

- - - Updated - - -

greg1313 said:
Alternatively,

$$y=mx+b$$

$$b=y-mx=\lim_{x\to\infty}\dfrac{x^3-5x^2+4x}{-4x^2+36}-\lim_{x\to\infty}-\dfrac x4$$

Simplify and evaluate the limit to find $b$. For other functions it may be necessary to evaluate $x\to-\infty$.

This is an exercise before limits are introduced
 
I still don't know what $b$ is?
 
Let's look at the long division:

$$\begin{array}{r}-\tfrac{1}{4}x+\tfrac{5}{4}\\-4x^2+0x+36\enclose{longdiv}{x^3-5x^2+4x+0} \\ -\underline{\left(x^3+0x^2-9x\right)} \hspace{22px} \\ -5x^2+13x+0 \\ -\underline{\left(-5x^2+0x+45\right)} \hspace{-12px} \\ 13x-45 \hspace{-5px} \end{array}$$

Thus, we may write:

$$\frac{x^3-5x^2+4x}{-4x^2+36}=-\frac{1}{4}x+\frac{5}{4}+\frac{13x-45}{-4x^2+36}$$

Now, notice the remainder (linear) over the divisor (quadratic) will tend to zero for values of $x$ having great magnitude, thus the oblique asymptote is the line:

$$y=-\frac{1}{4}x+\frac{5}{4}$$
 
Mark is correct; $$-\frac{1}{4}x+\frac{5}{4}$$.
When I do the long division, I usually like to verify my answer as such:
$$(-4x^2+36)(-\frac{1}{4}x+\frac{5}{4})+(13x-45)=x^3-5x^2+4x$$
 
i was curious about the latex for long division
 

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