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 February 28th, 2018, 03:14 AM #1 Newbie   Joined: Feb 2018 From: Diepenbeek, Limburg, Belgium Posts: 2 Thanks: 0 Trigonometry and a Convex Quadrilateral Hey, I'm stuck in another math problem... sin A + sin B + sin C + sin D = 4 * sin(A+B)/2 * sin(B+C)/2 * sin(C+A)/2 "prove in a convex square ABCD the formula above" thanks! Last edited by greg1313; February 28th, 2018 at 07:27 PM.
 February 28th, 2018, 04:53 PM #2 Global Moderator   Joined: May 2007 Posts: 6,823 Thanks: 723 What is a convex square?
 March 1st, 2018, 01:55 AM #3 Global Moderator   Joined: Dec 2006 Posts: 20,972 Thanks: 2222 It's reasonable to assume that "convex square" should be "convex quadrilateral". The given equation will hold if sin(A + B + C) = sin(D). This is easy to prove by use of these identities and the fact that the internal angles of a convex quadrilateral sum to 360°. Thanks from Fene

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