Negative 2 and two-thirds, negative 5 and one-third, negative 10 and two-thirds, negative 21 and one-third, negative 42 and two-thirds, ellipsis Which formula can be used to describe the sequence? f(x 1) = –2f(x) f(x 1) = Negative one-halff(x) f(x 1) = One-halff(x) f(x 1) = 2f(x).