Answers

2015-11-19T20:25:48+05:30
Sn=7+77+777+7777+……+to n terms            
  = (7/9) [9+99+999+……+ to n terms]               
= (7/9) [(10-1) + (100-1) + (1000-1) +……...+ to n term]               
= (7/9) [10+100+1000+……... n terms – (1+1+1+1+…… n terms)             
  = (7/9) [10+100+1000+………n terms –n]     
  a=10, r=10             
 = (7/9) [10(10 n-1)/ (9) - n]
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2015-11-19T20:33:45+05:30
Let Sn=7+77+777+7777.... Upto n terms. =7/9[9+99+999... n terms.] =7/9[(10-1)+(100-1)+(1000-1).. Upto n terms] =7/9[10+100+1000.. n terms] - [1+1+.. n terms] =7/9[{10(10^n - 1)/10-1} - n] =7/9[{10(10^n-1/9} - n] =70/81(10^n-1)-7n/9
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