Question 1198827
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First calculate f(2), using the given formula.


Next calculate f(3).


Finally, calculate f(4).



Alternatively, you may notice that the formula defines arithmetic progression 
with the 1st term  a(1) = 7 and the common difference  d = 2.


Then a(4) = a(1) + (4-1)*2,


and you can compute it mentally.
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