Question

In an A.P. Ist term is 1 and the last term is 20. The sum of all terms is = 399 then n=.. (A) 42 (B) 38 (C) 21 (D) 19 ​

Answer 1

Answer:

n = 38

OPTION b IS CORRECT

Step-by-step explanation:

$s = \frac{n}{2} (1 + 20) \\ \\ 399 = \frac{n}{2} (21) \\ \\ n = \frac{399 \times 2}{21} \\ \\ = 38.$