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The value of cos 180/11+ cos 3*180/11+cos 5*180/11+cos 7*180/11+cos 9*180/11 is 0 -1/2 1/2 1

The value of cos 180/11+ cos 3*180/11+cos 5*180/11+cos 7*180/11+cos 9*180/11 is



  1. 0

  2. -1/2

  3. 1/2

  4. 1


 

Grade:11

2 Answers

suryakanth AskiitiansExpert-IITB
105 Points
12 years ago

Dear abhipsa,

we know that

cos a + cos 3a + cos 5a + .. + cos(2n-1)a = sin 2na/2 sin a

In our case n = 5

therefore RHS = sin 10a / (2sina)

=>RHS =  sin 2*5 *(180/11) / 2sin (180/11)

=> RHS = sin (180/11) /2 sin(180/11)

=> rhs = 1/2

hence the answer is option 3 ......1/2

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anil dhar
8 Points
8 years ago
0

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