Question

Find the sum of the 1st 40 terms of the arithmetic sequence,-3,-7,-11..

Answer 1

First term, a = -3
Common difference, d = -7+3 = -4
$s40 = \frac{40}{2} (2a + (40 - 1) d) \\ = 20 \times (2 \times - 3 + 39 \times - 4) \\ = - 3240$