通分原理： 对于两个数x1y1,x2y2,\frac{x1}{y1},\frac{x2}{y2},y1x1 ,y2x2 , x1y1=x1y2y1y2,x2y2=x2y1y1y2\frac{x1}{y1}=\frac{x1y2}{y1y2},\frac{x2}{y2}=\frac{x2y1}{y1y2}y1x1 =y1y2x1y2 ,y2x2 =y1y2x2y1 所以我们只要比较x1y2x1y2x1y2与x2y1x2y1x2y1的大小就行了