Come risolvi #cos x - cos 2x = 0 #?

Usa la proprietà: #cos 2A=2cos^-2A-1#

#cosx-(2cos^2x -1)=0#

#-1 [cosx -2cos^2x+1]=0#

#2cos^2x-cosx-1=0#

#(2cosx+1)(cosx-1)=0#

#cosx=-1/2 or cos x=1#

#x=cos^-1(-1/2) or x=cos^-1 1#

#x=+- (2pi)/3 + 2pin or x=0+2pin#

#S={+- (2pi)/3 + 2pin , 0+2pin}#