y+（y-1）／2＝2-（y+2）/52-y=（y-1）/2+（y+2）/5（2-y）×10=（（y-1）/2+（y+2）/5）×1020-10y=5（y-1）+2（y+2）20-10y=5y-5+2y+420-10y=7y-117y＝21y=21/17
