y= - x的平方+6x+1=-(x²-6x+9)+10=-(x-3)²+10
y=2x的平方-3x+4=2(x²-3x/2+9/16)+23/8=2(x-3/4)²+23/8
y=-x的平方+nx=-(x²-nx+n²/4)+n²/4=-(x-n/2)²+n²/4
y=x的平方+px+q=(x²+px+p²/4)+q-p²/4=(x+p/2)²+q-p²/4