Answers

2016-07-14T13:05:11+08:00
Given:
a1 = 4
an = 80
d = 4
n = ?
sn = ?

Where...
a1 - first term that is divisible by 4
an - last term that is divisible by 4
d - common difference
n - number of terms
sn - sum of the terms

*The number of terms is not given. For solving the sum of the terms, we need to find n since it's required.

SOLVING FOR n:
an = a1 + (n - 1)d
80 = 4 + (n - 1) 4
80 = 4 + 4n - 4
4n = 80
n = 80 รท 4
n = 20

SOLVING FOR sn:
sn = n/2 (a1 + an)
sn = 20/2 (4 + 80)
sn = 10 (84)
sn = 840

The sum of the terms is 840.
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