Solve cos x + cos 2x = 0 ?

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Answer 1 · 2018-04-16T00:02:21

See below

#cosx+cos2x= 0#

Remember #cos2x= 2cos^2x-1#

Substitute:
#2cos^2x+cosx-1=0#

Factor like a quadratic:
#(2cosx-1)(cosx+1)= 0#

#cosx=-1#
#x=pi+2pin#

#cosx=1/2#
#x= pi/3+2pin#
#x= (5pi)/3+2pin#

Note #n# is an element of all integers

graph{cosx+cos(2x) [-10, 10, -5, 5]}