Question

In an A.P,the sum of first ten terms is -150 and the sum of next 10 terms is -550.Find the A.P

Answer 1

S10 = -150;
the sum of next 10 terms = S20 - S10 => -550 = S20 + 150 => S20 = -700;
S10 = -150 => 2a1 + 9r = -30;
S20 = -700 => 2a1 + 19r = -70;
Then 10r = -40 => r = -4 , and, 2a1 -36 = -30 => 2a1 = 6 => a1 = 3;