Tan-inv x + Tan-inv y + Tan-inv z

Tan-inv x + Tan-inv y + Tan-inv z = Tan-inv [(x+y+z-xyz)/(1-xy-yz-zx)]

Tan-inv x1 + Tan-inv x2 + ...+ Tan-inv xn = Tan-inv [(S1-S3+S5...)/(1-S2+S4-S6+...)]
Where Sx denotes the sum of the products of x1,x2,..,xn taken k at a time.