y = x^2-2ax +1
y' = 2x-2a =0
x= a
y'' = 2 >0 (min)
case 1: a ≤ 0
y > y(0) = 1
y < y(2) = 5-4a
值域 ( 1,5-4a)
case 2: 0min y = y(a)
=a^2-2a^2+1
= -a^2+1
y(0) = 1
y(2) = 5-4a(max)
值域 (-a^2+1, 5-4a)
case 3: a≥2
yy> y(2)=5-4a
值域 ( 5-4a, 1)