y=ax^2+bx+c(a≠0)
=a[x^2+2(b/2a)x]+c
=a[x^2+2(b/2a)x+(b/2a)^2]-a*(b/2a)^2+c
=a(x+b/2a)^2+(4ac-b^2)/4ac
所以顶点是(-b/2a,(4ac-b^2)/4ac)