Question

Find S20 for the arithmetic series 8 + 16 + 24 + 32 +…
super confused cause my options for answers are
1)1672
2)1688
3)1664
4)1680
shouldn't the answer be 160?

Answer 1

First you have to find the 20th term...
an = a1 + (n - 1)d
n = 20
a1 = 8
d = 8
now lets sub...
a20 = 8 + (20 - 1) * 8
a20 = 8 + 19 * 8
a20 = 8 + 152
a20 = 160

s20 is the sum of all the number till 20.
so we will use the sum formula...
sn = n(a1 + an) / 2
s20 = 20 (8 + 160) / 2
s20 = 20 (168) / 2
s20 = 3360/2
s20 = 1680

any questions ?