Hi,

What is the derivation of formula for sum of sine/cosine series, the angles of which are in A.P.?

Thanks,

Nishtha

Dear nishtha,

sin ( π + θ ) = –sinθ and sin (2π + θ ) = sinθ

                                       ._..  –sin (α + β) = sin(π +  )

.^..

  so on. Using these results, required sum is

  S=sin α+ … to n terms

    = sin α + sin ... to n terms

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