已知x2-3xy=-5,xy+y2=3,求x2-2xy+y2.
问题描述：
已知x2-3xy=-5,xy+y2=3,求x2-2xy+y2.
答
条件中所给的两个方程相加得x2-2xy+y2=-2
答
x2-2xy+y2=x2-3xy+xy+y2即-5+3=-2