How do you find S11 for the geometric series 1 + 3 + 9 + 27 +…?

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Answer 1 · 2015-12-25T22:29:16

The sum is: #S_11=88573#

To finf the sum you use the formula:

#S_n=a_1*(1-q^n)/(1-q)#

In this case you have:

#a_1=1#

#q=a_2/a_1=3/1=3#

#n=11#

so:

#S_11=1*(1-3^11)/(1-3)#

#S_11=(1-177147)/((-2))#

#S_11=-177146/(-2)#

#S_11=88573#