以这题为例
Y=X^2+6X+4
=(X^2+6X+3^2-3^2+4)
=(X+3)^2-5
顶点就为(-3,-5)
Y=AX方+BX+C
=A(X+B/2A)方+(4AC-B方)/4A
其实就是把顶点坐标直接代入呀