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Find the sum to n terms of the following...

Find the sum to n terms of the following series:
`cos alpha + cos (pi - alpha) + cos (2pi + alpha) + .......(0 < alpha < (pi)/2)`.

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The correct Answer is:
0 (when n is even and `cos alpha` (when n is odd)
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CHHAYA PUBLICATION-TRIGONOMETRIC RATIOS OF ASSOCIATED ANGLES-SAMPLE QUESTIONS FOR COMPETITIVE EXAMS - ASSERTION-REASON TYPE
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