Question

 

cosx*cos2x*cos3x=1

Answer 1

1/4 (cos(2 x)+cos(4 x)+cos(6 x)+1) = 1

 

TrigReduce[Cos[x] Cos[2 x] Cos[3 x] == 1] 

 

(1 + Cos[2 x] + Cos[4 x] + Cos[6 x])/4 == 1