2 cos pi/13cos 9pi/13+cos3pi/13+cos5pi/13=0
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Step-by-step explanation:
2cos pi/13 cos 9pi/13+ cos 3pi/13 +cos 5pi/13
=cos 10 pi/13 +cos 8 pi/13 +cos 3pi/13 +cos 5pi/13
=cos 10 pi/13 +cos 3pi/13 +cos 8pi/13 +cos 5pi/13
=2 cos pi/2 .cos 7 pi/26 +2 cos pi/2 .cos 3 pi /26
=2 (0)cos 7 pi /26 + 2(0) cos 3pi/26
=0 =R.H.S.
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