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December 3rd, 2016, 07:01 AM   #1
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Question 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 08:24 AM.
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December 3rd, 2016, 07:20 AM   #2
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$$2\sin{A}\, \cos{A}\, \sin{A} = 2 \cos{A}\, \sin{A}\, \sin{A} = 2 \cos{A}\, \sin^2{A} = 2\cos{A}\,(1-\cos^2{A})$$
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Last edited by v8archie; December 3rd, 2016 at 07:23 AM.
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