Calculate S5 for the sequence defined by {an}={1+5/2n}

13.5

23

40

42.5

This is an arithmetic series, where S5 means the sum of the first 5 numbers in the series. The general formula is:

Sn = (n/2) (a1 + an)

So,

a1 = 1 + 5/2 (1) = 3.5

a5 = 1 + 5/2 (5) = 1.5

S5 = (5/2) (3.5+1.5)

S5 = 12.5

The answer should be 12.5