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Question Number 165777 by ZiYangLee last updated on 08/Feb/22

$$\mathrm{The}\mathrm{sum}\mathrm{of}\mathrm{first}\mathrm{the}n\mathrm{terms}\mathrm{of}\mathrm{a}\mathrm{series}$$$$\left\{a_n\right\}\mathrm{is}\mathrm{given}\mathrm{by}{S}_n=3n^2+n,\mathrm{find}\underset{k=10}{\sum ^{20}}{a}_k.$$$$\mathrm{A})910$$$$\mathrm{B})913$$$$\mathrm{C})968$$$$\mathrm{D})1256$$

Answered by Rasheed.Sindhi last updated on 08/Feb/22

$$S_9=3{\left(9\right)}^2+9=252$$$$S_{20}=3{\left(20\right)}^2+20=1220$$$$\underset{k=10}{\sum ^{20}}{a}_k=S_{20}-S_9=1220-252=968$$

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