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$$f(x+y)=f(x)\cos y+f(y)\cos x$$

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$$f(x)=($BDj?t(B)\times\sin x$$

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$$g(x+y)=g(x)g(y)-\sin x\sin y$$

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$$\left\{ \begin{array}{lll} f(x+y) & = & f(x)g(y)+g(x)f(y)\\ g(x+y) & = & g(x)g(y)-f(x)f(y) \end{array} \right.$$

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