# If f(x) = cos x + cos ax is periodic function, then a must be

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If f(x) = cos x + cos ax is periodic function, then a must be

(A) an integer

(B) a rational number

(C) an irrational number

(D) an even number

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Correct Option:- (B) a rational number

Explanation :-

f(x) = cos x + cos ax

period of cos x = 2π