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by pfortuny 1366 days ago
However, if the symbolic package knows that

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

then it works.

Something must be known in advance about the relationship between sin and cos for addition, otherwise you cannot go from one to the other (and the basic

cos(a)^2+sin(a)^2=1

is not enough for that.