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prove that 2 cos(5*180/11) * cos(3*180/11) + cos (3*180/11) + cos(9*180/11)=0

prove that 2 cos(5*180/11) * cos(3*180/11) + cos (3*180/11) + cos(9*180/11)=0

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1 Answers

Yash Baheti IIT Roorkee
askIITians Faculty 97 Points
9 years ago
Hi,

Take 180/11 = k.
Use 2Cos(a)Cos(b) formulae. U will get Cos (8k) + Cos (2k).
Now combine 8k and 3k term and 2k and 9k term.
Apply Cos(c) + Cos(d) formula. You will get Cos(90) term in both the expressions, which will make the expressions zero. So the answer is 0.

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