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Using the fact that s in (A + B) = s in ...

Using the fact that `s in (A + B) = s in A cos B + cos A s in B`and the differentiation, obtain the sum formula for cosines.

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`sin(A+B)=sin A cos B +cos A sin B`
Taking A as a function of x and B as constant, differentiate w.r.t. x.
`cos(A+B)*(dA)/(dx)=cosB*cos A(dA)/(dx)+sinB(-sinA)(dA)/(dx)`
`implies cos(A+B)=cosAcosB-sinA sinB`
which is the required formula.
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NAGEEN PRAKASHAN ENGLISH-Continuity and Differentiability-Miscellaneous Exercise
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  15. If f(x)=|x|^3 , show that f"(x) exists for all real x and find it.

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