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I have a huge expression with (Co)Sine of a sum like Cos[[Theta] - 2 [Phi]] (after using \Simplify). I wanted to know if it exists a function which forces Mathematica to express the result in terms of multiplication of (Co)Sine instead of (Co)Sine of a sum ?

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Use Expand with the option Trig -> True, e.g., Expand[Cos[a + b], Trig -> True] gives Cos[a] Cos[b] - Sin[a] Sin[b] – Bob Hanlon Jul 9 '14 at 13:07
This answer should be very helpful: Checking if two trigonometric expressions are equal. Take a closer look at all examples therein. – Artes Jul 9 '14 at 13:23

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