What is the sum of the first 8 terms of the series: 11, 18, 25, 32...

We'll apply the formula of the sum of the n terms of an
arithmetic sequence:

Sn =
(a1+an)*n/2

Now, we'll substitute n by
8:

S8 = (a1 + a8)*8/2

S8 = (a1
+ a8)*4

We'll know the value of the first 4 terms from the
8 terms.

But we know the formula of finding any term of an
arithmetic series.

an  =a1 + (n-1)*d, where a1 is the first
term and d is the common difference.

a8 = a1 +
(8-1)*d

We could calculate d
from:

a2 - a1 = d

18 - 11 =
7

d = 7

a8 = 11 +
7*7

a8 = 11 + 49

a8 =
60

S8 = (11 + 60)*4

S8 =
71*4

S8 =
284

The sum of the first 8
terms of the arithmetic progression, whose first term is 11 and common difference is 7,
is S8 = 284.