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If alpha =(pi)/(18) radian then the valu...

If `alpha =(pi)/(18)` radian then the value of `(cos alpha + cos 2 alpha+ cos 3 alpha +…+ cos 18 alpha)` is-

A

`1 or, -1`

B

1

C

0

D

`-1`

Text Solution

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The correct Answer is:
D
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