Limits at Infinity, Part I — Question 9

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Question 9

Evaluate the limit: limx→−∞2x3+x2−5x3−4x+1\lim_{x \to -\infty} \frac{2x^3 + x^2 - 5}{x^3 - 4x + 1}

Original worksheet page 1: question and worked solution for 2-7-009
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Question 9 - Solution

We are given: limx→−∞2x3+x2−5x3−4x+1\lim_{x \to -\infty} \frac{2x^3 + x^2 - 5}{x^3 - 4x + 1}

Step 1: Factor out the highest power of xx (which is x3x^3) from both numerator and denominator:

=limx→−∞x3(2+1x−5x3)x3(1−4x2+1x3)= \lim_{x \to -\infty} \frac{x^3 \left(2 + \frac{1}{x} - \frac{5}{x^3} \right)}{x^3 \left(1 - \frac{4}{x^2} + \frac{1}{x^3} \right)}

Cancel the common x3x^3 terms:

=limx→−∞2+1x−5x31−4x2+1x3= \lim_{x \to -\infty} \frac{2 + \frac{1}{x} - \frac{5}{x^3}}{1 - \frac{4}{x^2} + \frac{1}{x^3}}

Step 2: Take the limit as x→−∞x \to -\infty. All terms with 1xn→0\frac{1}{x^n} \to 0:

=2+0−01−0+0=2= \frac{2 + 0 - 0}{1 - 0 + 0} = \boxed{2}

Original worksheet page 2: question and worked solution for 2-7-009

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