ចំនួនតេត្រាអែក (Tetrahedral Number)មានៈ 1, 4, 10, 20, 35, 56, 84, 120, 165, 220, 286, 364, 455, 680, 816, 969, ... រូបមន្តតួទី n របស់ចំនួននេះ៖ T n = n ( n + 1 ) ( n + 2 ) 6 = n 3 ¯ 3 ! {\displaystyle T_{n}={n(n+1)(n+2) \over 6}={n^{\overline {3}} \over 3!}} ឬ T n = ( n + 2 3 ) . {\displaystyle T_{n}={n+2 \choose 3}.}
