y=二分之一x²＋2x＋1=1/2(x+2)²-1
y=2x²-3x＋4 =2(x-3/4)²+23/8
y=-2x²＋4x-5=-2(x-1)²-3
y=x²-5x＋6=(x-5/2)²-1/4