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Question
Find the sum of the following
3, 7, 11, … up to 40 terms
Solution
3, 7, 11, ... upto 40 terms.
a = 3, d = t2 – t1 = 7 – 3 = 4
n = 40
Sn = `"n"/2` (2a + (n – 1)d)
S40 = `20/2` (2 × 3 + 39d)
= 20(6 + 39 × 4)
= 20(6 + 156)
= 20 × 162
= 3240
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