解由x^2+xy=-2xy+y^2=3两式相加得x^2+2xy+y^2=1故x的平方加2xy加y的平方=1.
解：x^2+2xy+y^2=x^2+xy+y^2+xy=-2+3=1
x2+2xy+y2=x2+xy+xy+y2=-2+3=1
