Question

The 14th term of an AP is twice its 9th term. If its 6th term is -8, then find the sum of its first 20 terms​

Answer 1

Answer:

-520

Step-by-step explanation:

a+5d=-8......(i)

=>a+13d=2(a+8d)

=>a+13d=2a+16d

=>a=-3d......(ii)

So,

-3d+5d=-8

=>2d=-8

=>d=-4

So,

a+5(-4)=-8

=>a-20=-8

=>a=12

20th term = a+19d=12+19(-4)=12-76=-64

So,

S20= n/2(a+l)

=20/2(12-64)

=10(-52)

=-520 (Ans)