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March 1st, 2018, 04:59 AM   #1
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proving a formula

Hello people
Can you help me with this??

prove the next formule in a convex quadrilateral ABCD:cos A+cos B+ cos C+ cos D = 4*cos (A+B)/2* cos (B+C)/2* cos (C+A)/2

Thanks!

Last edited by greg1313; March 1st, 2018 at 06:08 AM.
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March 1st, 2018, 06:08 AM   #2
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Last edited by greg1313; March 1st, 2018 at 09:20 AM.
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