y=kx+b与x轴交点的横坐标x3=-b/k
ax^2=kx+b
ax^2-kx-b=0
x1+x2=k/a
x1x2=-b/a
x1x2/(x1+x2)=-b/k=x3
所以:x1x2=x3(x1+x2)