1.

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

Answer» `S=7+77+777+7777..........n terms`
`S=7(1+11+111.......n terms)`
`S=7/9(9+99+999.......n terms)`
`S=7/9((10-1)+(100-1)+(1000-1).......n terms)`
`S=7/9(10+100+1000......n terms.-(1+1+1+1....n terms)`
`S=7/9( (10((10)^n-1))/(10-1)-n)`
`S=7/9( (10((10)^n-1))/(9)-n)`


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