Which of the following is identical to for every real ?

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

Problem

Which of the following is identical to cos⁡(x−π4)\cos\left(x-\dfrac{\pi}{4}\right) for every real xx? (a) sin⁡(x−π4)\sin\left(x-\dfrac{\pi}{4}\right) (b) cos⁡(x+π4)\cos\left(x+\dfrac{\pi}{4}\right) (c) sin⁡(x+π4)\sin\left(x+\dfrac{\pi}{4}\right) (d) sin⁡x+cos⁡x\sin x+\cos x

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