Mathematics

This is an arithmetic sequence problem—you need to recognize the pattern and test membership. Notice the sequence has a constant difference between consecutive terms; once you find that common difference $d$, you can write a formula for the $n$-th term. Then check whether 1999 fits that formula by solving for $n$—if $n$ is a positive integer, 1999 is in the sequence; if not, it isn't.

This is an arithmetic sequence problem—you need to recognize the pattern and test membership. Notice the sequence has a constant difference between consecutive terms; once you find that common difference $d$, you can write a formula for the $n$-th term. Then check whether 1999 fits that formula by solving for $n$—if $n$ is a positive integer, 1999 is in the sequence; if not, it isn't.