Question
Mon July 30, 2012 By:

prove that

Expert Reply
Mon July 30, 2012
cos 4A
= cos 2(2A)
= 1 - 2 sin22A
= 1 - 2 (2sinA cosA)2
= 1 - 8 sin2A cos2A
= 1 - 8 sin2A (1- sin2A)
= 1 - 8 sin2A + 8 sin4A
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