



 
Ali, If a function f(x) is periodic with period k then for any x, f(x + k) = f(x). So for example if the period is k = 3 and f(2) = 7 then f(5) = f(2 + 3) = f(2) = 7 and also f(8) = f(5 + 3) = f(5) = 7. You can also find f(11) since 11 = 8 + 3 ad so on. So let's try a problem that's similar to the one you sent.
Since the period is 7 and we know the value f(2) = 4 then
But we jumped over f(17) which is what we want. So let's start with f(3) = 12.
But that is what we want so I know that f(17) = 12. Now try the problem you sent us,  


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