Mathematics

Mathematics

Problem

Which of the following identities holds true for all real xx and yy? a) cos⁡(x+y)=cos⁡xcos⁡y+sin⁡xsin⁡y\cos(x+y)=\cos x\cos y+\sin x\sin y b) sin⁡(x+y)=sin⁡xcos⁡y−cos⁡xsin⁡y\sin(x+y)=\sin x\cos y-\cos x\sin y c) cos⁡(x+y)=cos⁡xcos⁡y−sin⁡xsin⁡y\cos(x+y)=\cos x\cos y-\sin x\sin y d) sin⁡(x+y)=sin⁡xsin⁡y+cos⁡xcos⁡y\sin(x+y)=\sin x\sin y+\cos x\cos y

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