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 ?
(Moodle 3.11.3 ; Stack 4.3.8 ; Goemaxima 2020120600)