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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


1 Answer

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by (68.1k points)
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Best answer

Correct Option:- (B) a rational number

Explanation :-

f(x) = cos x + cos ax 

period of cos x = 2π

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