Find the sum of sequence, 7, 77, 777, 7777..... to n terms ???

plz ans fast....

its very urgent

= 7+77 +777+----+n

=7(1+11+111+----+n)

Multiplying and dividing 9

7/9 ( 9+99+999------+n)

=7/9 (10-1+100-1+1000-1------+n)

=7/9 ( 10-1+10²-1+10³-1-------+n)

Take all multiples of 10 together to form a GP and put all the -1's separate-

=7/9( 10+10²+10³....-n) (since there are 'n' terms so there are' n' no. of -1's so n times -1(n*-1) = -n)

Using formula for sum of GP

=7/9 { 10( 10ⁿ - 1)/9 - n}

= 70/81(10ⁿ-1) - n