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Essential Calculus Early Transcendentals
Explain why each of the following integrals is improper.
$$\int_{1}^{2} \frac{x}{x-1} d x \quad \text { (b) } \int_{0}^{\infty} \frac{1}{1+x^{3}} d x$$
$$\int_{-\infty}^{\infty} x^{2} e^{-x^{2}} d x \quad \text { (d) } \int_{0}^{\pi / 4} \cot x d x$$