If the members of the set :
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.