Question

已知x—xy=40,xy—y=—20,求代数式x—y和x+y—2xy的值

Answer 1

(x-xy)-(xy-y)=40-(-20)

x-xy-xy+y=40+20

x+y-2xy=60

(x-xy)+(xy-y)=40+(-20)

x-xy+xy-y=40-20

x-y=20