Question

What is the sum of the first 25 terms of an arithmetic sequence with 20 as the first term and d=-2?​

Answer 1

Ans : -100

Step-by-step explanation:

a = 2 , d= -2

$sum \: of \: first \: n \: terms \: of \: an \: ap \: = \frac{n}{2} (2a + (n - 1)d) \\ sum \: of \: first \: 25 \: terms \: of \: an \: ap = \frac{25}{2} (2(20) + (25 - 1) - 2) \\ = \frac{25}{2} (40 + 24( - 2)) \\ = \frac{25}{2} (40 - 48) \\ = \frac{25}{2} ( - 8) \\ = 25 \times - 4 \\ = - 100$