Asked by Arissa
By using the formula cos (A+B) = cos A cos B - sin A sin B , prove that cos A - cos 3A = 4 sin^2 A cos A
Answers
Answered by
oobleck
well, how far did you get?
cos3A = cos(2A+A) = cos2A cosA - sin2A sinA
= (1-2Sin^2A)(cosA) - (2SinAcosA)(sinA)
try and finish it up from here
cos3A = cos(2A+A) = cos2A cosA - sin2A sinA
= (1-2Sin^2A)(cosA) - (2SinAcosA)(sinA)
try and finish it up from here
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