© 2018 danted2015

Week 2-Geometric Series

How to find S_{n}

If you are given with the geoettic series of

80 – 40 + 20 – …               and it asks to find S_{6}

Find the r

Which is -2

Then put all the numbers in the equation:

S_{6}=\frac{a(r^8 -1)}{r-1}

Let’s go:

S_{6}=\frac{80(-2^6 -1)}{-2-1} S_{6}=\frac{80(63)}{-3} S_{6}=\frac{5040}{-3} S_{6}=-1680




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