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Abhipsa Sadangi Grade: 11
        

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


 

6 years ago

Answers : (2)

suryakanth AskiitiansExpert-IITB
105 Points
										

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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6 years ago
anil dhar
8 Points
										
0
2 years ago
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