Question

Find the sum of series 7+77+777 to the nth term

Answer 1

Step-by-step explanation:

Sn=7+77+777+7777+……+to n terms            

  = (7/9) [9+99+999+……+ to n terms]               

= (7/9) [(10-1) + (100-1) + (1000-1) +……...+ to n term]               

= (7/9) [10+100+1000+……... n terms – (1+1+1+1+…… n terms)             

  = (7/9) [10+100+1000+………n terms –n]     

  a=10, r=10             

 = (7/9) [10(10 n-1)/ (9) - n]