2、设随机变量X的分布函数为 (x)=a+barctan x, 则常数a,b的值为 ()-|||-(A) =dfrac (1)(2), =dfrac (1)(pi )-|||-(B) =dfrac (1)(2) ,=-dfrac (1)(pi )-|||-(C) =-dfrac (1)(2) ,=dfrac (1)(pi )-|||-(D) =-dfrac (1)(2) ,=-dfrac (1)(pi )