Question

已知:x-xy=40 xy-y=-20 求:x+y=?是初一的题,

Answer 1

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,自己算算吧...最后好象是解一元二次方程)即解得a就是x+y的值.