f(x) = ln(1 + x) - 2x/(x + 2)
f'(x) = 1/(1 + x) - 2[(x + 2)(x') - x(x + 2)']/(x + 2)²
= 1/(1 + x) - 2[(x + 2) - x]/(x + 2)²
= 1/(1 + x) - [2(x + 2) - 2x]/(x + 2)²
导数商法则：
(u/v)' = (vu' - uv')/v²