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Naveen Sharma 8 years, 10 months ago
Ans. Sn = 8( 1 + 11 + 111 + 1111 + ..... Upto n terms )
=> 8/9 (9 + 99 + 999 + 9999 + ……… upto n terms)
=> 8/9 [ (10-1) + (100-1) + (1000-1) + upto n terms]
=> 8/9 [ (10 + 100 + 1000 + ...... n terms ) -(1+ 1 + 1 + ………… n terms)]
=> 8/9 [ 10(10n -1)/ (10-1) - n ]
=> 8/9 [ 10(10n-1)/9 - n]
=> 80(10n -1) /81 - 8n/9
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