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2014-08-12T21:56:49+03:00
F(1)=4*1-1=4-1=3
f(2)=4*2-1=8-1=7
f(3)=4*3-1=12-1=11
...............................
f(10)=4*10-1=40-1=39

S=3+7+11+...+39=  (3+39)*10/2=  42*5= 90
3 3 3