设x1,x2,则x1+x2=-b/a,x1*x2=c/a.由已知得x1=-3x2,分别代入得2x2=b/a,-3x2^2=c/a,左边弄成相等式得b^2/a^2*(-3/4)=c/a,整理得-3b^2=4ac