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[tex]S_{1}=11^{1}+11^{2}+11^{3}+...+11^{x}=>a_{1}=11^{1};\ r=11=>S_{215}=a_{1}\cdot\frac{1-r^{215}}{1-r}=>S_{215}=11\cdot\frac{1-11^{215}}{1-11}=>S_{215}=\frac{11^{216}-11}{10}[/tex]

deci 11+11^2+11^3+...+11^215=(11^216-11)/10

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