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The value of cos4 x + sin4 x – 6 cos2 x sin2 x isA. cos 2xB. sin 2xC. cos 4xD. None of these |
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Answer» Correct answer is A. Given expression is cos4 x + sin4 x – 6 cos2 x sin2 x = [(cos2x)2 + (sin2x)2 - 2 cos2x sin2x] - 4 cos2x sin2x [using the formula a2 + b2 = (a + b)2 - 2ab] = (cos2x - sin2x)2 - 4 cos2x sin2x [using the formula cos 2x = cos2 x – sin2 x] = (cos2x)2 – (2 sinx cosx )2 [ using the formula sin 2x = 2 sin x cos x ] = (cos 2x)2 – (sin 2x)2 [ using the formula cos 2x = cos2 x – sin2 x ] = cos 4x Therefore cos4 x + sin4 x – 6 cos2 x sin2 x = cos 4x |
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