Question

The 14th term of an A.P is twice it's 18th term.If it's 6th term is -8 the find the sum of it's first n term.

Answer 1

I hope it's clear. And someone verify if it is correct.

Answer 2

let the first term be a
and the common diff be d
according to question
a+5d=-8 equation 1
now
14th term=2×18th term
a+13d=2a+34d
a+21d=0 equation
now bt subtracting equation 1 and 2 we get
d=0.5


a+5d=-8
a=-8-2.5
a=-10.5


sum of nth term=n/2(2a+(n-1)d)
=n/2(2×-10.5 +0.5n-0.5)
n/2(-21 +0.5n)