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1 vote
4 + 12 + 36 + … ; n = 15

2 Answers

1 vote
I've attached a picture below of the formula for a finite geometric sequence. It includes an infinite geometric sequence formula for your reference, too.


a_1 = 4

r(ratio)= 3

n=15

Plug in the values.


S_(15) = 4*( (1-3^(15))/(1-3) )

S_(15) = 4*( (1-3^(15))/(-2) )

S_(15) = 4*( (1-14348907)/(-2) )

S_(15) = 4*( (-14348906)/(-2) )

S_(15) = 4*7174453
S₁₅ = 28697812
4 + 12 + 36 + … ; n = 15-example-1
User Mohan Ramanathan
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6.1k points
4 votes
4 times 3=12
12 times 3=36
I think you want us to find the sum of the geometric sequence to n=15

the sum is

S= (a_1(1-r^(n)))/(1-r)
a1=first term=4
n=15
r=common ratio=3


S= (4(1-3^(15)))/(1-3)

S= (4(1-14348907))/(-2)
S=-2(-14348906)
S=28697812

it adds to 28697812
User Srneczek
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5.4k points