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# CBSE Class 11-science Answered

The following questions may kindly be answered.

Asked by tps.mjmdr | 16 Jun, 2018, 08:37: PM Expert Answer
cosθ + cos(∏+θ) + .....+cos(n∏+θ)
Consider,
cos(∏+θ) = cos∏cosθ - sin∏sinθ
As sinn∏ is always zero and cos∏ = -1
cos(∏+θ) = -cosθ
similarly cos2∏ = 1
Now we have two cases
i) n is odd
then cos(n∏+θ) = (-1)n cosθ = -cosθ
ii) n is even
then cos(n∏+θ) = (-1)n cosθ = cosθ
USe this and solve it further
Answered by Sneha shidid | 19 Jun, 2018, 11:04: AM

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