s= 7+77+777+..
can we take 7 as common the we get
7(1+11+111...)
therefore we will get the sum of 1+11+111...
as
1*10^(n-1)+2*10^(n-2)...
this is ap gp series so we can solve it
but it is getting tedious
also i dont know whether this is right or wrong
please help