If a1, a2, ..........an+1 are in A.P with common difference d, then \(\sum\limits_{r =1}^n tan^{-1}\frac d{1 + a_ra_{r + 1}}\)
(a) \(tan^{-1} \frac{nd}{1 + a_1a_{n+1}}\)
(b) \(tan^{-1} \frac{(n +1)d}{1 + a_1a_{n+1}}\)
(c) \(tan^{-1} \frac{(n -1)d}{1 - a_1a_{n+1}}\)
(d) \(tan^{-1} \frac{(n -1)d}{1 + a_1a_{n+1}}\)