Question

For the arithmetic sequence beginning with the terms {-2, 0, 2, 4, 6, 8...}, what is the sum of the first 18 terms?

Answer 1

Answer:

The sum of the first 18 terms is 270.

Step-by-step 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