Is there is quicker way to find the solutions to cos(x) = - cos(3x)?

Is there is quicker way to find the solutions to cos(x) = - cos(3x)?
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Answer :

answer:3x = pi – x + 2npi or 3x = pi + x + 2npi x = (2n+1)pi/4 or x=(2n+1)pi/2
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