Answers

2015-03-19T11:35:23+05:30
7+77+777+.............................+n
=7(1+11+.............n)
=7(1+10+100+......)+7n
which is a GP(1+10+.....)
the sum is a(r^n-1/r-1)
=7((10^n -1/9 )+n)+7n
=7{(10^n-1)/9 +n}
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2015-03-19T11:37:20+05:30
The ratio between two consecutive terms is 11,
so according to the GP 

Sn={a(r^n-1)}/(r-1)

Sn={7(10^n-1}/10

Thats all Folks!!!
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