Question

若x^2+xy+5y3,y^2+xy+5x=-3,求x+y的值

Answer 1

x^2+xy+5y=3,y^2+xy+5x=-3

两式相加得：x^2+2xy+y^2+5(x+y)=0,即：(x+y)^2+5(x+y)=0,(x+y)(x+y+5)=0

所以x+y=0或x+y= -5