Question

Find the sum of sequence 7,77,777,777,
- to n terms.​

Answer 1

7, 77, 777, 7777 ….to n terms.         

Sn=7+77+777+7777+……+to n terms      

introducing 9       

  = (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]     

 Here      a=10, r=10             

 = (7/9) [10(10n-1)/ (9) - n]

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