Answer :
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}
=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}
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!!!
so according to the GP
Sn={a(r^n-1)}/(r-1)
Sn={7(10^n-1}/10
Thats all Folks!!!