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Answer:
f[5] = 9
Step-by-step explanation:
We can work this one term at a time.
f[3] = 2f[2] -f[1] = 2(3) -(1) = 5
f[4] = 2f[3] -f[2] = 2(5) -3 = 7
f[5] = 2f[4] -f[3] = 2(7) -5 = 9
_____
Additional comment
The terms known to this point seem to indicate f[n] = 2n -1 is a workable simplification. Or, recursively, f[n] = f[n-1] +2.
