Which of the following identities holds true for every real angle ?

MATHEMATICS · Single correct · A Level · Mathematics · Trigonometry · Trigonometry

Problem

Which of the following identities holds true for every real angle θ\theta? (a) sin⁡θ+cos⁡θ=1\sin\theta+\cos\theta=1 (b) sin⁡θcos⁡θ=tan⁡θ\dfrac{\sin\theta}{\cos\theta}=\tan\theta (c) sin⁡2θ+cos⁡2θ=1\sin^{2}\theta+\cos^{2}\theta=1 (d) sin⁡2θ−cos⁡2θ=1\sin^{2}\theta-\cos^{2}\theta=1

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