x1+x2=-b,x1x2=c.1x3+x4=-b^2,x3x4=20.2x2-x3=x1-x4=3,则x1+x2-（x3+x4）=6,即-b+b^2=6,得b=3或b=-2由2式得b=3时有解,x3=-4,x4=-5,或x3=-5,x4=-4所以x1=-2,x2=-1或x1=-1,x2=-2所以c=2所以二次函数是y=x^2+3x+2顶点坐...由2式得b=3时有解,为何b=-2无解x3=20/x4x3+x4=20/x4+x4>=4根号5>(-2)^2=4所以x3+x4=4不可能b=-2无解