x的平方+xy=12,xy+y的平方=13,两式相加,得x²+2xy+y²=25,则(x+y)²=25,两式相减,得x²-y²=-1,则(x-y)(x+y)=-1,则(x-y)²(x+y)²=1则(x-y)²=1/25则x²+y²=[(x+y)²+...