Find Sn, the sum of the first n terms, for the following geometric series. a1=120,a5=1,r=−2.
A
20.66
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B
40.66
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C
80.66
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D
100.66
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Solution
The correct option is B40.66 Given, first term, a=120, common ratio, r=−2 and a5=1 We know Sn=a1−anr1−r Sn=120−(−2)1−(−2) Sn=120−(−2)1−(−2) Sn=1223 Sn=40.66