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If A+B +C +D= 2 pi, prove that : cos A +...

If `A+B +C +D= 2 pi`, prove that : `cos A + cos B + cos C + cos D =-4 cos frac (A+B)(2) cos frac (A+C)(2) cos frac (A+D)(2)`.

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MODERN PUBLICATION-TRIGNOMETRY-EXERCISE
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