1. Find the sum of the …
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Gaurav Seth 5 years ago
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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