等号对齐

Question

我解答了一道题，并用 Latex 写出来，但等号没有对齐，我在谷歌上搜索，但没有找到适合我的方法。如何让下面的等号对齐？

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代码：

\item Let $C$ be the boundary of the rectangle $[0,1]\times [3,4].$ Use Green's theorem to find $\oint_{C}{\bf{F}}\cdot d{\bf{s}},$ where ${\bf{F}}=[x-y,x^2].$
Solution
    $$\oint_{C}{\bf{F}}\cdot d{\bf{s}} =\iint_{R}\frac{\partial f_{2}}{\partial x}-\frac{\partial f_{1}}{\partial y}$$\\
    $$=\int^{1}_{0}\int^{4}_{3}2x-1 \,dydx$$
    $$=\left.\int_{0}^{1}2xy-y\right]^{4}_{3} \,dx$$
    $$=\int_{0}^{1}\left(8x-4\right)-\left(6x-3\right) \,dx$$
    $$=\int_{0}^{1}2x-1 \,dx$$
    $$=\left.x^{2}-x\right]_{0}^{1}$$
    $$=\left(1-1\right)-\left(0-0\right)$$
    $$=0$$\\

Answer 1

您应该使用amsmath及其align*环境。

请注意其他修复：标点符号应位于内联数学公式之外；该命令\bf已过时好几年了，\mathbf应该是首选；\left. <terms> \right]对于积分的评估最好使用\Bigr]（或其他大小）；使用该\diff命令，微分的排版更好、更容易。

\documentclass{article}
\usepackage{amsmath}

\newcommand{\diff}{\mathop{}\!d}

\begin{document}

\begin{enumerate}

\item Let $C$ be the boundary of the rectangle $[0,1]\times [3,4]$.
      Use Green's theorem to find $\oint_{C}{\bf{F}}\cdot \diff\mathbf{s}$,
      where ${\bf{F}}=[x-y,x^2]$.

Solution
\begin{align*}
\oint_{C}\mathbf{F}\cdot\diff\mathbf{s} 
 &= \iint_{R}\frac{\partial f_{2}}{\partial x}-\frac{\partial f_{1}}{\partial y} \\
 &= \int^{1}_{0}\int^{4}_{3}2x-1 \diff y\diff x \\
 &= \int_{0}^{1}2xy-y\Bigr]^{4}_{3} \diff x \\
 &= \int_{0}^{1}(8x-4)-(6x-3) \diff x \\
 &= \int_{0}^{1}(2x-1) \diff x \\
 &= x^{2}-x\Bigr]_{0}^{1} \\
 &= (1-1)-(0-0) \\
 &=0
\end{align*}

\end{enumerate}

\end{document}