The first few terms of a sequence (xn) are given below. Assuming that the ''natural pattern'' indicated by these terms persists, give a formula for the nth term xn.
(a) 5, 7, 9, 11, . . . ,
(b) 1/2, -1/4, 1/8, -1/16, . . . ,
(c) 1/2, 2/3, 3/4, 4/5, . . . ,
(d) 1, 4, 9, 16, . . . .