I am doing an investigation about a game called triminoes (like dominoes). The game is played using triangular pieces of card. Each card has 3 numbers on it. I have to investigate the relationship between the number of trimino cards in a set and the largest number on the cards. I found; largest no. used   0  1  2  3  4
 no. of trimino cards  1  4  10 20 35
  I was ginen the formula for this which is:
 UN= UN - 1 + 1/2 (n + 1 ) (n+2) 
   UN=no. of trimino cards  n= largest no. 
 I don't know how to get to this equation I think it has something to do with triangle numbers!