The triangular numbers are the following: T = 1,T = 3, T3 = 6, T4 = 10, ... They are called triangular numbers because you can think of the number as a series of dots which can be arranged into a triangle as shown below: T-1 T-3 T3-6 The formula for the nth triangular number is Tn is 10, so T = 10. 2628 is a triangular number. Which one is it? 74-10 n(n+1) = 2 -. For example, the fourth triangular number For example, if 2628 happened to be the 4th triangular number (which of course it's not), your answer would be 4.