g(x)= 0 x不等于0
6 x=0
f(x)= 2 x不等于0
3 x=0
f(g(x))= 3 x不等于0
2 x=0
lim g(x)=0 limf(y)=2 limf(g(x))=3
x->0 y->0 x->0