a = 2n
and (n-1)d = 20
so (2n-2)d = 40
(a-2)d = 40
So we could pick
a=3, d=40
check:
3,43,83,...3+40(n-1) = n/2 (6+(n-1)40)
= 3n + 20n^2 - 20n = n(20n-17)
But there are many other possibilities.
The sum of the first n terms of an arithmetic sequence is n/2 (4n+20).
Find the first term and the common difference of the above sequence.
1 answer