y=2x^2-8x+1=2(x^2-4x)+1=2(x-2)^2-8+1=2(x-2)^2-7
顶点为(2,-7),最小值为-7
y=-x^2+2x+4=-(x^2-2x)+4=-(x-1)^2+1+4=-(x-1)^2+5
顶点为(1,5),最大值为5