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find the sum of the geometric series

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3-6+12-.... to 7 terms

PLEASE HELP ME!

asked Mar 7, 2013 in ALGEBRA 1 by anonymous Apprentice

1 Answer

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Geometric series is 3-6+12-.... to 7 terms

First term,a=3

Common ration,r=-6/3=-2

Number of terms,n=7

Recall the formulae for sum of the n terms in Geometric series

Sn=a*(1-r^n)/1-r)       (r<1)

Substitute 7 for n,3 for a and -2 for r

S7=3*(1-(-2)^7)/1-(-2))  

  =3(1-(-128))/3

 =129

 

Sum of the 7 term is 129

 

I hope that helps u

answered Mar 7, 2013 by bradely Mentor
edited Mar 7, 2013 by bradely

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