定义域为：x>-1
f(x)=ln(x+1)-x
f '(x)=1/(x+1)-1= (-x)/(x+1)
令f '(x)>0==>x 定义域为：x>-1
f(x)=ln(x+1)-x
f '(x)=1/(x+1)-1= (-x)/(x+1)
令f '(x)>0==>x