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2014-08-10T12:27:51+03:00
F(1)= 3*1 -2= 3-2=1
f(2)=3*2 -2= 6-2=4
f(3)=3*3-2= 9-2=7
...........................
f(10)=3*10-2=30-2=28

S=(1+28)*10/2=  29*5=  145