y=ax^2 +bx+c=a(x^2+b/a *x)+c= a(x^2+b/a*x +b^2/4a^2 - b^2/4a^2)+c=a(x+b/2a)^2 -b^2/4a + c=a(x+b/2a)^2 -b^2/4a + 4ac/4a=a(x+b/2a)^2+(4ac -b^2) /4a 所以顶点坐标是（-b/2a ,(4ac -b^2) /4a )
