I want to use the trigonometric identity:
Sin(x)*Sin(y)=Cos(x-y)/2 - Cos(x+y)/2
With the expression e1=Sin[a]Sin[a+b]
I want to write a rule transformation that would get an expression and exchange it.
my code is:
Sin[x]*Sin[y] -> Substract[Divide[Cos[x - y], 2], Divide[Cos[x + y], 2]]
The problem is that Mathematica doesent recognize e1 as a pattern of the type Sin[x]Sin[y]