根据以往经验,感觉题目没抄完整,少了个-xy,所以补充了.
如果不是这样,别借鉴
x^2-xy=21,xy-y^2=-12
x^2-y^2=（x^2-xy）+(xy-y^2)=21+(-12)=9
x^2-2xy+y^2=(x^2-xy)-(xy-y^2)=21-(-12)=33