## 率计算公式
```math
\displaystyle
\frac{a}{b}=\frac{c}{d}\Rightarrow ad=bc\\{}\\
\Rightarrow a=\frac{bc}{d};\ b=\frac{ad}{c};\\{}\\
\frac{a}{c}=\frac{b}{d};\ \frac{d}{b}=\frac{c}{a};\ \frac{d}{c}=\frac{b}{a};\\{}\\
\frac{a\pm b}{b}=\frac{c\pm d}{d};\\{}\\
\frac{a+b}{a-b}=\frac{c+d}{c-d};\\{}\\
\frac{a\pm b}{a}=\frac{c\pm d}{c};\\{}\\
\frac{a}{a\pm b}=\frac{c}{c\pm d};\\{}\\
\frac{b}{a\pm b}=\frac{d}{c\pm d}
```