Concept:The function is periodic with period a, so the integral over each interval of length a is the same.Explanation:Given f(x)=f(x+a), the function repeats every a units.The integral over one period is ∫0af(x)dx=k.The interval [0,na] contains exactly n complete periods of length a.Therefore, ∫0naf(x)dx=n⋅∫0af(x)dx=nk.Answer:∫0naf(x)dx=nk