y=x^2+2ax+2
f'(x) =2x+2a=0
x =-a
f''(x)=2 > 0 (min)
case 1: if a > 2
max f(x) = f(-1)
y(-1) = 1-2a+2 = 4
a =-1/2
no solution for case 1:
case 2 if -1≤a≤2
f(2) = 8+4a =4 => a = -1
f(-1) = 3-2a = 4 => a = 1/2
solution for case 2: a=1/2 or -1
case 2: a a= -1
no solution for case 3
a的值: a=1/2 or -1