Question

The 14th term of an A.P is twice its 8th term. If its 6th term is -8, then find the sum of
its first 20 terms?

Answer 1

Answer:

a14 = 2×a8

a+13d = 2(a+7d)

a+13d = 2a + 14d

a-2a = 14d-13d

-a = 1d

a = -1d

a6 = a+5d

-8 = -d + 5d

-8 = 4d

d = -8/4 = -2

a= -1(-2) = 2

S20 = n/2 (2a+19d)

S20 = 20/2 (2×2 +19(-2))

S20 = 10 (4-38)

S20 = 10(-34)

S20 = -340