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f1+f2=4f2

f2=f1/3

f1+f2+f3=9f3

f1+f1/3+f3=9f3

f3=f1/6

f1+f2+f3+f4=16f4

We get f4=f1/10

So we can say fn=f1/sum of n terms

therefore f9=3600/45=80
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