Question

Find S15 for the geometric series 72 + 12 + 2 + +…

Answer 1

This is what we just did, but notice what is happening to the terms. What is r here?

Answer 2

72-12=60
12-2=10

Answer 3

Geometric means we multiply or divide to go from one term to the next so.....

Answer 4

R=6

Answer 5

1/6, we are dividing to go from one term to the next. See?

Answer 6

So the equation is
72(1-6^15)/(1-6)

Answer 7

\[72 (\frac{ 1 - (\frac{ 1}{ 6 })^{15} }{ 1-\frac{ 1 }{ 6 } })\]

Answer 8

Careful calculating. Do it one little piece at a time. What calculator are you using?

Answer 9

A Computer Calculator

Answer 10

You answer will be a "nice" number. Not a ton of decimal places.