Geometric series SUM

L

Louise Johnson

Junior Member
Joined
Jan 21, 2007
Messages
103
Hello I have two questions I have answered . The first went okay using the formula however the second I am uncertain about. Thanks Louise

Question #1
Find the sum of the first nine terms of the geometric series
-2+14+(-98)+686-......

My answer: a=-2 r=-7
S9=a(r^9-1)/r-1
-2(-7^9-1)/-7-1 =-10088402

Question #2
Without solving for the number of terms,determine the sum of the series below using a formula. Do not round your answer. 144+72+36+......+0.5625

My answer: a=144 r=1/2
S=a/1-r
S=144/1-1/2
S=288
 
1st one ok; not 2nd one: 288 is sum to infinity; sum to .5625 = 287.4375
 
Thank you for taking a look. I can see what you mean Dennis by my answer being to infinity. Can I ask what formula you used to get 287.4375?? In the mean time I will try and figure out how you got it. (my calculator spits out 288 already rounded?)
Thank you
Louise
 
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