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Evaluate the limit: lim (x->-infinity) x2 * e^x

Mathematics

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Answer

0\mathbf{0}

Step-by-step solution

5 steps
  1. Identify the form of the limit.

    As x→−∞x \to -\infty, exe^x approaches 0, and x2x^2 approaches ∞\infty.
    The limit is of the indeterminate form 0×∞0 \times \infty.

  2. Rewrite the expression to apply L'Hôpital's Rule.

    To use L'Hôpital's Rule, we need the expression in the form 00\frac{0}{0} or ∞∞\frac{\infty}{\infty}. We can rewrite ex×x2e^x \times x^2 as x2e−x\frac{x^2}{e^{-x}}.

  3. Apply L'Hôpital's Rule for the first time.

    As x→−∞x \to -\infty, the numerator x2→∞x^2 \to \infty and the denominator e−x→e−(−∞)=e∞=∞e^{-x} \to e^{-(-\infty)} = e^{\infty} = \infty. This is an ∞∞\frac{\infty}{\infty} form.

    lim⁡x→−∞x2e−x=lim⁡x→−∞ddx(x2)ddx(e−x)=lim⁡x→−∞2x−e−x\lim_{x \to -\infty} \frac{x^2}{e^{-x}} = \lim_{x \to -\infty} \frac{\frac{d}{dx}(x^2)}{\frac{d}{dx}(e^{-x})} = \lim_{x \to -\infty} \frac{2x}{-e^{-x}}
  4. Apply L'Hôpital's Rule for the second time.

    As x→−∞x \to -\infty, the numerator 2x→−∞2x \to -\infty and the denominator −e−x→−e−(−∞)=−∞-e^{-x} \to -e^{-(-\infty)} = -\infty. This is still an ∞∞\frac{\infty}{\infty} form.

    lim⁡x→−∞2x−e−x=lim⁡x→−∞ddx(2x)ddx(−e−x)=lim⁡x→−∞2−(−e−x)=lim⁡x→−∞2e−x\lim_{x \to -\infty} \frac{2x}{-e^{-x}} = \lim_{x \to -\infty} \frac{\frac{d}{dx}(2x)}{\frac{d}{dx}(-e^{-x})} = \lim_{x \to -\infty} \frac{2}{-(-e^{-x})} = \lim_{x \to -\infty} \frac{2}{e^{-x}}
  5. Evaluate the final limit.

    As x→−∞x \to -\infty, e−xe^{-x} approaches e−(−∞)=e∞=∞e^{-(-\infty)} = e^{\infty} = \infty.
    Therefore, the expression 2e−x\frac{2}{e^{-x}} approaches 2∞\frac{2}{\infty}, which is 0.

    lim⁡x→−∞2e−x=0\lim_{x \to -\infty} \frac{2}{e^{-x}} = 0
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