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If P=cos(pi/19)+cos((3pi)/19)+cos((5pi)/...

If `P=cos(pi/19)+cos((3pi)/19)+cos((5pi)/19)+....+cos((17pi)/19)` and `Q=cos((2pi)/21)+cos((4pi)/21)+....cos((20pi)/21)` then find `P-Q`

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ALLEN-TRIGONOMETRIC RATIOS AND IDENTITIES-All Questions
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