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निम्न श्रेणी के n पदों का योग ज्ञात कीजिए | `0.7+0.77+0.777`+…

Answer» माना `S=0.7+0.77+0.777`+...
`=7[0.1+0.11+0.111+..]`
`=(7)/(9)[0.9+0.99+0.999+]`
`=(7)/(9)[(1-(1)/(10))+(1-(1)/(100))+(1-(1)/(1000))+...]`
`=(7)/(9)[(1+1+1+...)-((1)/(10)+(1)/(100)+(1)/(1000)+..)]`
`=(7)/(9)[n-(1)/(10)((1-(1)/(10^(n))))/((1-(1)/(10)))]=(7n)/(9)-(7)/(81)(1-(1)/(10^(n)))`


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