Sum of first n the terms in the following series Cot-7 + cot-13 + cot-3 + cot-21 + ........... is given by:

(1) tan^-1(n/n+2) (2) Cot^-1 (n+2/n) (3) tan^-1 (n+1) - Tan^-1 1 (4) all of these