Question

7+77+777+7777+............+nterms

Answer 1

7+77+777+7777+........+n terms
taking out 7 common
7(1+11+111+1111................)
now multiplying by $\frac{9}{9}$
$\frac{7}{9}$(9+99+999+9999..............+n terms)
$\frac{7}{9}$[(10-1)+(100-1)+(1000-1)..........................]
$\frac{7}{9}$[(10+100+1000+10000..........)-(1+1+1+1+1+1.........n)]
$\frac{7}{9}$[$\frac{10( 10^{n}-1)}{10-1}$-n]
now u can further solve it

Answer 2

Answer:

see the attachment for the solution.