Answer:
The sum of the first 18 terms is 270.
Explanation:
First the 18th term is calculated which is gotten using the following steps below:
an = a1 + d ( n - 1 )
Where, an =18th term
a1= first term (-2)
d = difference
n = 18
Therefore, 18th term
an= -2 + 2 (18 - 1)
= -2 + 2 (17)
= -2 + 34
18th term = 32
To calculate the sum of the first 18th terms the following steps are used,
Sn = n(a1 + an)/2
Where,
Sn = sum of 18th term
n = 18
a1 = -2
an = 18 term
Therefore,
Sn = 18 (- 2+ 32)/2
= 18 (30)/2
= 18 ×30 /2
= 540/2
= 270