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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

Answer»

To find: cos4A – sin4

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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