f（3x+1）的定义域为【1,4】
1<=x<=4
4<=3x+1<=13
所以f(x)定义域[4,13]
所以f(x²)则4<=x²<=13
-√13<=x<=-2,2<=x<=√13
f(x+3)则4<=x+3<=13
1<=x<=10
同时成立
所以定义域[2,√13]