18. 求 ∫_1^∞ ( x)/x^2 dx = ? (A) $1$ (B) $0$ (C) $e$ (D) 2

24. 令 $f(x, y)$ 為雙變數函數定義如下： $$f(x, y) = \begin{cases} \frac{xy(x^2 - y^2)}{x^2 + y^2}, & \text{當}(x, y) \neq (0, 0) \\ 0, & \text{當}(x, y) = (0, 0) \end{cases}$$ $$\text{則 } \frac{\partial^2 f}{\partial x \partial y} (0, 0) - \frac{\partial^2 f}{\partial y \partial x} (0, 0) = ?$$ (A) $-2$ (B) $0$ (C) $1$ (D) $2$