align是输入多行公式中最好用的环境，仅仅是个人浅见，因为他的对齐非常灵活，如果大家需要非常灵巧的对齐方式的多行公式，建议使用align环境，对应的也还有align*和aligned等等类似的环境，这里不再详述。下文提供代码，尽展其风姿绰约。
演示效果图：
演示代码：
documentclass{article}
pagestyle{empty}
setcounter{page}{6}
setlength extwidth{266.0pt}
usepackage{CJK}
usepackage{amsmath}
egin{CJK}{GBK}{song}
egin{document}
egin{align}
(a + b)^3 &= (a + b) (a + b)^2 \
&= (a + b)(a^2 + 2ab + b^2) \
&= a^3 + 3a^2b + 3ab^2 + b^3
end{align}
egin{align}
x^2 + y^2 & = 1 \
x & = sqrt{1-y^2}
end{align}
This example has two column-pairs.
egin{align} ext{Compare }
x^2 + y^2 &= 1 &
x^3 + y^3 &= 1 \
x &= sqrt {1-y^2} &
x &= sqrt[3]{1-y^3}
end{align}
This example has three column-pairs.
egin{align}
x &= y & X &= Y &
a &= b+c \
x' &= y' & X' &= Y' &
a' &= b \
x + x' &= y + y' &
X + X' &= Y + Y' & a'b &= c'b
end{align}
This example has two column-pairs.
egin{flalign} ext{Compare }
x^2 + y^2 &= 1 &
x^3 + y^3 &= 1 \
x &= sqrt {1-y^2} &
x &= sqrt[3]{1-y^3}
end{flalign}
This example has three column-pairs.
egin{flalign}
x &= y & X &= Y &
a &= b+c \
x' &= y' & X' &= Y' &
a' &= b \
x + x' &= y + y' &
X + X' &= Y + Y' & a'b &= c'b
end{flalign}
This example has two column-pairs.
enewcommandminalignsep{0pt}
egin{align} ext{Compare }
x^2 + y^2 &= 1 &
x^3 + y^3 &= 1 \
x &= sqrt {1-y^2} &
x &= sqrt[3]{1-y^3}
end{align}
This example has three column-pairs.
enewcommandminalignsep{15pt}
egin{flalign}
x &= y & X &= Y &
a &= b+c \
x' &= y' & X' &= Y' &
a' &= b \
x + x' &= y + y' &
X + X' &= Y + Y' & a'b &= c'b
end{flalign}
enewcommandminalignsep{2em}
egin{align}
x &= y && ext{by hypothesis} \
x' &= y' && ext{by definition} \
x + x' &= y + y' && ext{by Axiom 1}
end{align}
egin{equation}
egin{aligned}
x^2 + y^2 &= 1 \
x &= sqrt{1-y^2} \
ext{and also }y &= sqrt{1-x^2}
end{aligned} qquad
egin{gathered}
(a + b)^2 = a^2 + 2ab + b^2 \
(a + b) cdot (a - b) = a^2 - b^2
end{gathered} end{equation}
egin{equation}
egin{aligned}[b]
x^2 + y^2 &= 1 \
x &= sqrt{1-y^2} \
ext{and also }y &= sqrt{1-x^2}
end{aligned} qquad
egin{gathered}[t]
(a + b)^2 = a^2 + 2ab + b^2 \
(a + b) cdot (a - b) = a^2 - b^2
end{gathered}
end{equation}
ewenvironment{rcase}
{left.egin{aligned}}
{end{aligned}
ight
brace}
egin{equation*}
egin{rcase}
B' &= -partial imes E \
E' &= partial imes B - 4pi j \,
end{rcase}
quad ext {Maxwell's equations}
end{equation*}
egin{equation} egin{aligned}
V_j &= v_j &
X_i &= x_i - q_i x_j &
&= u_j + sum_{i
e j} q_i \
V_i &= v_i - q_i v_j &
X_j &= x_j &
U_i &= u_i
end{aligned} end{equation}
egin{align}
A_1 &= N_0 (lambda ; Omega')
- phi ( lambda ; Omega') \
A_2 &= phi (lambda ; Omega')
phi (lambda ; Omega) \
intertext{and finally}
A_3 &= mathcal{N} (lambda ; omega)
end{align}
end{CJK}
end{document}
from: http://blog.sina.com.cn/s/blog_5e16f1770100gror.html