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Prove that cos4θ – sin4θ = 2cos2θ – 1 |
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Answer» L.H.S. cos4θ – sin4θ = (cos2θ)2 – (sin2θ)2 = (cos2θ + sin2θ) (cos2θ – sin2θ) = 1 . (cos2θ – (1 – cos2θ) = cos2θ – 1 + cos2θ = 2cos2θ – 1 = R.H.S. |
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