S={2

^{x}3

^{y}: x>=0,y>=0}

are arranged in increasing order we get the sequence beginning

1,2,3,4,6,8,9,12,16,18,......

(a)What is the position of 2

^{a}3

^{b}in the sequence in terms of a and b.

(b)What is the n-th term of the sequence in terms of n.