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cos4A − sin4A is equal to A. 2 cos2A + 1 B. 2 cos2A − 1 C. 2 sin2A − 1 D. 2 sin2A + 1 |
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Answer» To find: cos4A – sin4A Consider cos4A – sin4A = (cos2A)2 – (sin2A)2 ∵ a2 – b2 = (a – b) (a + b) ∴ cos4A – sin4A = (cos2A)2 – (sin2 A)2 = (cos2A – sin2A) (cos2A + sin2A) = (cos2A – sin2A) [∵ cos2A + sin2A = 1] = cos2A –(1 – cos2A) [∵ sin2A = 1 – cos2A] = cos2A – 1 + cos2A = 2 cos2A – 1 |
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