1.

Find the sum of the series : (i) `8+88+888+` ... to n terms (ii) `3+33+333+` ... to n terms (iii) `0.7+0.77+0.777+`... to n terms

Answer» Correct Answer - (i) `8/81[10^((n+1))-10-9n]` (ii) `1/27 [10^((n+1))-10-9n]` (iii) `7/81(9n-1+1/10^(n))`
(i) Given sum `=8xx[1+11+111+..."to n terms"]`
`=8/9xx[9+99+999+..."to n terms"]`
`=8/9xx[(10-1)+(10^(2)-1)+(10^(3)-1)+..."to n terms"]`
`=8/9xx{(10+10^(2)+10^(3)+..."to n terms")-n}`
`=8/9 xx{10xx((10^(n)-1)/(10-1))-n}=8/81 [10^((n-1))-10-9n]`.
(iii) Given series `=7/10+77/100+777/1000+..."to n terms"`
`=7xx{1/10+11/100+111/1000+..."to n terms"}`
`=7/9xx{9/10+99/100+999/1000+..."to n terms"}`
`=7/9xx{(1-1/10)+(1-1/10^(2))+(1-1/10^(3))+..."to n terms"}`
`7/9xx{n-(1/10+1/10^(2)+1/10^(3)+..."to n terms")}`
`=7/9xx{n-(1/10(1-1/10^(n)))/((1-1/10))}=7/81xx(9n-1+1/10^(n))`.


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