已知x的平方-xy=40,xy-y的平方=-20求代数式x的平方-y的平方和3x的平方-5xy+2y的平方的值

x^2-xy=40(1)
xy-y^2=-20 (2)
(1)+(2)得x^2-xy+xy-y^2=x^2-y^2=40-20=20
3x^2-5xy+2y^2=3x^2-3xy - 2xy+2y^2=3(x^2-xy)-2(xy-y^2)将(1)(2) 代入
=3×40-2×(-20)=120+40=160