f(x) = ln(1+x) -mx
f'(x) = 1/(1+x) - m
f'(1) =0
m = 1/2
f''(x) = - 1/(1+x)^2 < 0 ( for x≠ -1 )
f(x) is defined for x > -1
f(x)的单调区间
增加 (-1,1/2)
减小(1/2,∞)