Question

Sum of the first 10 terms of the arithmetic sequence -15, -7, 1, 9?

Answer 1

Given sequence:

-15, -7, 1 ,9,....

a (first term) = -15

d (common difference ) = -7 -(-15) = -8

n (no. of terms) = 10

sum of first 10 terms;

$s = \frac{n}{2} \times (2a + (n - 1)d)$

putting values in the formula;

$s = \frac{10}{2} (2 \times - 15 + (10 - 1) \times -8)$

$s = 5( - 30 + 9 \times -8)$

$s = 5( - 30 -72)$

$s = 5(102)$

$s = 510$