Cosπ\5+ cos2π\5+ cos6π\5+cos7π\5
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Answer:
0.
Explanation:
cos 180/5+ cos 2*180/5+ cos 6*180/5+ cos 7*180/5
=cos 36+cos 72+ cos 216+ cos 252
=cos 36+ cos 72+ cos(180+36)+ cos (180+72)
=cos 36+ cos 72+(-cos 36)+(-cos 72){Reason: (180+36) lies in third quadrant where cos is negative}
=cos 36+ cos 72- cos 36- cos 72
=0 ( proved)...
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