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OpenStudy (anonymous):
Use summation notation to write each arithmetic series, evaluate the sum. 21+19+17+...n=8
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OpenStudy (anonymous):
a=21,d=19-21=-2 sn=8 \[sn=\frac{ n }{ 2 }\left\{ 2a+\left( n-1 \right)d \right\}\] calculate n .
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