y=ax＊2+bx+c
y=a(x＊2+b/a•x)+c
y=a[x＊2+b/a•x+(b/(2a))＊2-(b/(2a))＊2]+c
y=a[(x+b/(2a))＊2-(b/(2a))＊2]+c
y=a(x+b/(2a))＊2-a•b＊2/(4a＊2)+c
y=a(x+b/(2a))＊2-b＊2/(4a)+4ac/(4a)
y=a[x+b/(2a)]＊2+(4ac-b＊2)/(4a)