1. A student needs to evaluate $\int x^3 e^{-2x}\,dx$. They set up a tabular method table. Which of the following correctly gives the first two terms of the result?
- A.$-\frac{x^3}{2}e^{-2x} + \frac{3x^2}{4}e^{-2x} + \cdots$
- B.$-\frac{x^3}{2}e^{-2x} - \frac{3x^2}{4}e^{-2x} + \cdots$
- C.$\frac{x^3}{2}e^{-2x} - \frac{3x^2}{4}e^{-2x} + \cdots$
- D.$-\frac{x^3}{2}e^{-2x} + \frac{3x^2}{2}e^{-2x} + \cdots$
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Answer: $-\frac{x^3}{2}e^{-2x} - \frac{3x^2}{4}e^{-2x} + \cdots$
Set up the tabular method: Derivatives of $x^3$: $x^3, 3x^2, 6x, 6, 0$. Antiderivatives of $e^{-2x}$: $-\frac{1}{2}e^{-2x}, \frac{1}{4}e^{-2x}, -\frac{1}{8}e^{-2x}, \frac{1}{16}e^{-2x}$. Signs: $+, -, +, -$. Compute the first two terms: First term: $(+)(x^3)(-\frac{1}{2}e^{-2x}) = -\frac{x^3}{2}e^{-2x}$. Second term: $(-)(3x^2)(\frac{1}{4}e^{-2x}) = -\frac{3x^2}{4}e^{-2x}$. Why distractors fail: Option A has a plus for the second term (wrong sign from the alternating pattern). Option C has a plus for the first term (wrong). Option D has $\frac{3x^2}{2}$ instead of $\frac{3x^2}{4}$.