Saved Bookmarks
| 1. |
Ex. 8 Find 7+77+777+.... upto n terms |
|
Answer» 7, 77, 777, 7777 ….to n terms. Sn=7+77+777+7777+……+to n termsintroducing 9 = (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] Here a=10, r=10= (7/9) [10(10n-1)/ (9) - n] |
|