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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: 7,308 Thanks: 2443 Math Focus: Mainly analysis and algebra 
$$2\sin{A}\, \cos{A}\, \sin{A} = 2 \cos{A}\, \sin{A}\, \sin{A} = 2 \cos{A}\, \sin^2{A} = 2\cos{A}\,(1\cos^2{A})$$
Last edited by v8archie; December 3rd, 2016 at 06:23 AM. 

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