Question

In an ap, the sum of first ten terms is -150 and the sum of its next ten terms is-550, find the ap

Answer 1

Answer:

S10 =10/2 (2a +9d)

-150 = 5 (2a+9d)

2a+9d = -30 ....(1)

and

S20- S10 = 20/2 (2a+19d) - 10/2(2a+9d)

-550 = 10 ( 2a+19d )- 5(2a +9d)

-110 = 2( 2a+19d )-(2a +9d)

-110 = 4a + 38d - 2a - 9d

-110 = 2a +29d ....(2)

subtract (1) from (2)

we get,

20d = -80

d = -4

put this in any of the above eqn

we get,

2a = 6

a = 3

required AP is 3, -1, -5, -9,-13.....

hope this will help you