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- 4 sin2 (A+B4. Prove the following(i) cos 35°+cos 85° +cos 155° 0 |
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Answer» Cos 35°+cos85°+cos155°= 2cos{(35°+85°)/2} cos{(35°-85°)/2}+cos(180°-25°)[cosC+cosD=2cos{(C+D)/2}cos{(C-D)/2}]=2cos(120°/2)cos(50°/2)+cos{(90°×2)-25°}=2cos60°cos25°+(-cos25°)=2×(1/2)cos25°-cos25°=0 (Proved) |
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