问题
[$\textcolor{Orange}{\int \frac{1}{1 + x^{2}} \mathrm{d} x}$] 的积分该怎么计算？选项
[A]. $\int$ $\frac{1}{1 + x^{2}}$ $\mathrm{d} x$ $=$ $\arctan \frac{1}{x}$ $+$ $C$[B]. $\int$ $\frac{1}{1 + x^{2}}$ $\mathrm{d} x$ $=$ $\arctan x$
[C]. $\int$ $\frac{1}{1 + x^{2}}$ $\mathrm{d} x$ $=$ $\arctan x$ $+$ $C$
[D]. $\int$ $\frac{1}{1 + x^{2}}$ $\mathrm{d} x$ $=$ $\arctan x^{2}$ $+$ $C$
$$\int \textcolor{Red}{\frac{1}{1 + x^{2}}} \mathrm{d} x =$$ $$\textcolor{Red}{\arctan x} + C$$