Question

In an arithmetic series, what is the sum of the first 22 terms if the first term is 12 and the common difference is 3?

Answer 1

561

Explanation:

an = dn - (a-d)

The difference is 3.

The first term is 12.

an = 3n - (12-3)

an = 3n - 9

Put n as 1, 2, 3, 4, 5 ....22.

3(1) - 9 = -6

3(2) - 9 = -3

3(3) - 9 = 0

3(4) - 9 = 3

3(5) - 9 = 6

...

3(22) - 9 = 57

Add the first 22 terms.

-6+-3+0+3+6+9+12+15+18+21+24+27+30+33+36+39+42+45+48+51+54+57

= 561