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Find the sum of first forty positive integers divisible by 6.

Solution:

The first forty positive integers are 0, 1, 2, 3, 4, …
and the numbers divisible by 6 will be 6, 12, 18, 24, … to 40 terms
Here, a = 6, d = 12 – 6 = 6, n = 40. Then we have: