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cos 2θ cos 2ϕ + sin2 (θ – ϕ) – sin2 (θ + ϕ) is equal to …..(a) sin 2(θ + ϕ) (b) cos 2(θ + ϕ) (c) sin 2(θ – ϕ) (d) cos 2(θ – ϕ) |
Answer» (b) cos 2(θ + ϕ) Given cos 2θ . cos 2ϕ + sin2 (θ – ϕ) – sin2 (θ + ϕ) = cos 2θ cos 2ϕ + sin (θ – ϕ + θ + ϕ) sin (θ – ϕ – θ – ϕ) = cos 2θ cos 2ϕ + sin 2θ sin(-2ϕ) = cos 2θ cos 2ϕ – sin 2θ sin(2ϕ) = cos (2θ + 2ϕ) = cos 2(θ + ϕ) |
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