is(cos4A-sin4A) is equal to cos2A
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cos^4 A - sin^4 A = (1 + cos 2A / 2)^2 - (1 - cos 2A / 2)^2 [double angle formulae]
= (1 + 2 cos 2A + cos^2 2A - 1 + 2 cos 2A - cos^2 2A) / 4
= 4 cos 2A / 4
= cos 2A
= (1 + 2 cos 2A + cos^2 2A - 1 + 2 cos 2A - cos^2 2A) / 4
= 4 cos 2A / 4
= cos 2A
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