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sin(a+b)=sin(a)cos(b)+cos(a)sin(b)

d/dx (f(x)*g(x))=f(x)g'(x)+g(x)f'(x)

Somewhere on this very site there was mention of this, I believe, though I can't remember where. Maybe I'm delirious.

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# The sin(a+b) formula and the product rule for derivatives

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