x-xy=40 (1) xy-y=-20(2) (1)+(2)得：x-xy+xy-y=40-20=20 即x-y=20则:(x-y)^2=(x+y)^2-4xy=400(1)-(2)得:x-xy-xy+y=40-(-20)=60 即x+y-2xy=60令x+y=a,xy=b 则a^2-4b=400,a-2b=60已知方程组可求结果(很难算,自己算算...