这需要技巧的：
lim(x → 0) sin3x / ln(1 + 3x)
= lim(x → 0) 3x / ln(1 + 3x) * lim(x → 0) sin3x / 3x
= lim(x → 0) 1 / ln[(1+3x)^(1 / 3x)] * lim(x → 0) sin3x / 3x
= 1 / ln(e) * 1,定理lim(x → 0) (1 + x)^(1 / x) = e
= 1