Hi,

I'm working with the trigonometric relation cos(a)cos(b) = 0.5cos(a+b) + 0.5cos(a-b).

Some of my questions work and other don't.

For example, the following algebraic equivalence is correct :

cos(2*x)*cos(20*x) <==> 0.5*cos(18*x)+0.5*cos(22*x)

But there is a error with the following :

cos(20*x)*cos(200*x) and 0.5*cos(180*x)+0.5*cos(220*x)

Do you know why ?

Regard,

Vincent (France)

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