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Trigonometric Identity Proofs Question

[Deleted User] · 2009-05-30 14:19:50

If the question for the proof of, just say, sin2A = 2sinAcosA came up, are you allowed to just sub A in for B into the sin(A+B) = sinAcosB+cosAsinB formula? Like I know that's how it's derived, but, are you allowed to assume that the sin(A+B) formula is true? Or do you've to derive that before you use it (as it's proof is…

30-05-2009 2:19pm

#1

[Deleted User]
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If the question for the proof of, just say, sin2A = 2sinAcosA came up, are you allowed to just sub A in for B into the sin(A+B) = sinAcosB+cosAsinB formula? Like I know that's how it's derived, but, are you allowed to assume that the sin(A+B) formula is true? Or do you've to derive that before you use it (as it's proof is on the syllabus, too).

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#2 30-05-2009 2:34pm

Fringe

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Look on the log tables. You are allowed to use the formulae before that one to prove it. sin2A is after sin(A + so yes, you can use that to prove it.

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#3 30-05-2009 3:00pm

[Deleted User]
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Oh right right, I didn't know that about the order. Thanks Fringe.

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