1.

If \[{{a}_{1}},\ {{a}_{2}},............,{{a}_{n}}\] are in A.P. with common difference , \[d\], then the sum of the following series is \[\sin d(\cos \text{ec}\,{{a}_{1}}.co\text{sec}\,{{a}_{2}}+\text{cosec}\,{{a}_{2}}.\text{cosec}\,{{a}_{3}}+...........\]\[+\text{cosec}\ {{a}_{n-1}}\text{cosec}\ {{a}_{n}})\] [RPET 2000]

A. \[\sec {{a}_{1}}-\sec {{a}_{n}}\]
B. \[\cot {{a}_{1}}-\cot {{a}_{n}}\]
C. \[\tan {{a}_{1}}-\tan {{a}_{n}}\]
D. \[c\text{osec}\ {{a}_{1}}-\text{cosec}\ {{a}_{n}}\]
Answer» C. \[\tan {{a}_{1}}-\tan {{a}_{n}}\]


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