Question

What is the sum of the arithmetic sequence 3, 9, 15..., if there are 36 terms?

Answer 1

The common difference is 6.
Now, plug this into the equation for the sum of an arithmetic sequence:

`(n)/(2)[2a(1) + (n-1)d]`
a(1) is 3 (the first number)
n = 36 (the number of terms to sum up to)
d = 6 (common difference)

`18[6 + (35*6)]`

`18[216]`
3888