" 19."11y=tan^(-1)((a_(1)x-a)/(a_(1)a+x))+tan^(-1)((a_(2)-a_(1))/(1+a_(2)))+tan^(1)((a_(2)-a_(2))/(1+a_(2)a_(3)))+cdots+tan^(2)((a_(2))/(1+a_(2)))+cdots