45)
The first step is to find the three terms, a1, a2 and a3 by substituting i = 1, 2 and 3 respectively into the formula given. We have
a1 = 5 * 1 + 3 = 5 + 3 = 8
a2 = 5 * 2 + 3 = 10 + 3 = 13
a3 = 5 * 3 + 3 = 15 + 3 = 18
The formula for calculating the sum of the first n terms of an arithmetic sequence is expressed as
Sn = n/2[2a + d(n - 1)]
where
Sn is the sum of the first n terms
n is the number of terms
a is the first term
d is the common difference
From the information given,
a = 8
d = 13 - 8 = 18 - 13 = 5
n = 40
Thus,
S40 = 40/2[2 * 8 + 5(40 - 1)]
S40 = 20[16 + 195]
S40 = 4220
The indicated sum is 4220