Question

Find the sum of the first 20 terms of the AP series 2 + 7 + 12 + using the suitable formula

Answer 1

Step-by-step explanation:

given that :

first term = 2

common difference = 7 - 2 = 5

nth term = 20

Then according to questions :

$sn = \frac{n}{2} (2a + (n - 1)d) \\ \\ = \frac{20}{2} (2 \times 2 + (20 - 1)5) \\ \\ = 10(4 + 19 \times 5) \\ \\ = 10(4 + 95) \\ \\ = 10 \times 99 \\ \\ = 990 \\ \\ sn = 990$