请教一道高一集合题，谢！

3a+1<2, a < 1/3时，A = {x| 3a+1 < x < 2}.
3a+1 <= 2a, a <= -1;
a^2 + 1 <= 2, -1 <= a <= 1.
a = -1.

3a+1>2, a > 1/3时，A = {x| 2 < x < 3a+1}
2 <= 2a, a >= 1.
a^2 + 1 <= 3a+1, a(a-3) <= 0, 0 <= a <= 3.
1 <= a <= 3.

3a+1 = 2, a = 1/3时，A 是空集。而B永远不是空集，B永远不可能是A的子集。

综合，有
a = -1或1<=a<=3时，B是A的子集。

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