sisorayi01 wrote:

Its probably obvious.. but can someone explain why the formula for a arithmetic series is used here ... Sum of series n = (n/2)(A1 - An) ... as opposed to the formula for a geometric series ... Sum of series n = [A1*(1-r^n)]/(1-r) where r is the common ratio and n is the nth term.

I understand the solution but am confused by the formula thinking that An=4n+1 is a geometric sequence given that you have to multiply by 4 to get the next term in the sequence. Appreciate your reply!!

THANKS!

Adding to what Bunuel just posted, a geometric series would look like following

An = 4^n

And the series would look something like 4, 16, 64, 256, 1024, ...