求指导本题解题过程，谢谢您！11.已知当x→0时, (x)=arctan x-sin ax 与 (x)=bxln sqrt (a+{x)^2} 是等价无穷小,则-|||-(A) =b=1. (B) =2, =dfrac (1)(3).-|||-(C) =1, =dfrac (1)(2). (D) =1, =-dfrac (1)(3),

求指导本题解题过程，谢谢您！