(a)2,4,6,8,10,12,       the even positive integers, an = 2n
 (b)5,10,15,20,25,30     multiples of 5, an =  5n
 (c)1,4,9,16,25,36       squares of the positive integers, an = n² 
 (d)10,20,30,40,50,60    multiples of 10, an = 10n
 (e)2,5,10,17,26,37      add 1 to each term of (c), an = n²+1
 (f)18,27,36,45,54       multiples of 9 starting with 9×2, an = 9(n+1)
 (g)2,9,28,65,126,217    add 1 to each cube, an = n³+1
 (h)4,7,10,13,16,19      add 1 to each multiple of 3, an = =3n+1

Edwin