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 Trigonometry Trigonometry Math Forum

 December 3rd, 2016, 06:01 AM #1 Newbie   Joined: Dec 2016 From: Ireland Posts: 4 Thanks: 0 Trig Identities (please help!) In the solution to a question, it says that 2sinAcosA.sinA is equal to 2cosA(1 - cos^2A). How are they equal? I can't figure it out! (Note that the question I am doing is to prove that cos3A = 4cos^3A - 3cosA.) Last edited by skipjack; December 3rd, 2016 at 07:24 AM.
 December 3rd, 2016, 06:20 AM #2 Math Team   Joined: Dec 2013 From: Colombia Posts: 7,341 Thanks: 2463 Math Focus: Mainly analysis and algebra $$2\sin{A}\, \cos{A}\, \sin{A} = 2 \cos{A}\, \sin{A}\, \sin{A} = 2 \cos{A}\, \sin^2{A} = 2\cos{A}\,(1-\cos^2{A})$$ Thanks from Aoife73 Last edited by v8archie; December 3rd, 2016 at 06:23 AM.

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