x^2-xy=12,且xy+y^2=13
x^2+y^2
=(x^2-xy)+(xy+y^2)
=12+13
=25
或者是x^2+xy=12,且xy+y^2=13
求x^2-y^2
=(x^2+xy)-(xy+y^2)
=12-13
=-1
题目好像抄错了一点