Question

Write a formula for the sum of the first n terms in this series

Answer 1

d = 9-1 = 8

$S_n= \frac{2+8(n-1)}{2}*n= \frac{2(1+4(n-1))}{2}*n=(1+4n-4)n=(4n-3)n= \\\\=4n^2-3n$

The answer is A) 4n²-3n

Answer 2

The formula for the sum of an arithmetic series with first term "a" and common difference "d" is
.. Sn = (d*n² +n*(2a -d))/2

You have a=1, d=8, so your sum is
.. Sn = (8n² +n*(2 -8))/2
.. Sn = 4n² -3n . . . . . . . . . . . matches selection A