x²-2ax+(a²-1)≤0
x²-2ax+a²≤1
(x-a)²≤1
-1≤x-a≤1
a-1≤x≤a+1
B={x|a-1≤x≤a+1},
B包含于 A
a+1≤4 ,a-1≥1
a≤3,a≥2
2≤a≤3