i cant find 12 distinct combinations if N = 2...
i can only think of:
0-0
0-1
0-2
1-1
1-2
2-2
That are only 6 combinations... what did i miss?

0-0 (0+0=0)
0-1 (0+1=1)
0-2 (0+2=2)
1-1 (1+1=2)
1-2 (1+2=3)
2-2 (2+2=4)
         |
0+1+2+2+3+4=12

got it?

The problem does not ask you to count the # of distinct dominoes in a set, it asks you to count the # of dots on a complete set.

but how does the formula N*(N+1)/2*(N+2) comes ? pls explain... :)