Question

What is the sum of the geometric sequence 1, 3, 9, … if there are 12 terms?

292,524

265,720

139,968

104,976

Answer 1

ooh easy

Answer 2

ar^0+ar^1+ar^2... +ar^11 the rule
1 2 3 ... 12 term
1 3 9 ? result

a=1
r=3

(1-3^12)/(1-3)=265720
^ here before i had 12 lol.

It was just a typo

Answer 3

found a and r this way.

since ar^0= a =1 a is 1

so we look at 2nd term
ar^1=3 it just reduces to
r=3

now we use the geometric series formula
http://www.purplemath.com/modules/series5.htm


(1-3^12)/(1-12)=48312.727272 <- with the error