anonymous
  • anonymous
What is the sum of the geometric sequence -3, 18, -108, if there are 7 terms?
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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.Sam.
  • .Sam.
First find the common ratio
.Sam.
  • .Sam.
Then use \[\Large S_N=a_1\frac{1-r^N}{1-r}\] Where r is common ratio and N is the number of terms and \(a_1\) is the frist term
anonymous
  • anonymous
How do you find the common ratio?

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.Sam.
  • .Sam.
The second term divided by the first term
.Sam.
  • .Sam.
or, the third term divided by the second term
anonymous
  • anonymous
so it would be -3 divided by 18?
.Sam.
  • .Sam.
\[r=\frac{18}{-3}=-6 \\ \\ r=\frac{-108}{18}=-6\]
anonymous
  • anonymous
so 3(1-(-6)7/1-(-6) ??
.Sam.
  • .Sam.
You forgot the minus sign for 3
.Sam.
  • .Sam.
\[S_7=-3\frac{1-(-6)^7}{1-(-6)}\]
anonymous
  • anonymous
Do we figure out the exponent first or minus 6 from 1 first
.Sam.
  • .Sam.
Exponent first, always.
anonymous
  • anonymous
the answer is -119973 right?
.Sam.
  • .Sam.
Yep
anonymous
  • anonymous
Thankyouu so muchhh...this was correct!

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