algebra

[sud21]

Sequence s1,s2,s3,..sn, is such that s=1/n-1/n+1 . Is the sum of first n terms greater than 9/10?
1). n>10
2). n<19

[mankey]

IMO:A.

Sum reduces to 1-{1/n+1}. For this sum to be greater than 9/10, n>9.

First one tells us this, second doesnt guarantee. So A.

What is the OA?

Thanks.

[user123321]

Sequence s1,s2,s3,..sn, is such that s=1/n-1/n+1 . Is the sum of first n terms greater than 9/10?
1). n>10
2). n<19

1) given S = 1/n - 1/n+1
=> in order for the sum to be greater than 9/10 we need to have atleast 11 terms...because S10 = 9/10
so if n> 10 is a proper condition hence sufficient.
2) n<19
=> sum can be <9/10 or can be >9/10. hence insufficient.

[spoiler]IMO A?[/spoiler]

user123321