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9π13 13ST2coSCOScoS+ COS01313

Answer»

2cos π/13 cos 9π/13+ cos 3π/13 +cos 5π/13

=cos 10 π/13 +cos 8 π/13 +cos 3π/13 +cos 5π/13

=cos 10 π/13 +cos 3π/13 +cos 8π/13 +cos 5π/13

=2 cos π/2 .cos 7 π/26 +2 cos π/2 .cos 3 π /26

=2 (0)cos 7 π /26 + 2(0) cos 3π/26

=0 =R.H.S.



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