Question: how do you prove a number is not triangular?
A number N is a triangular number if N (or Tn) = n(n+1)/2,
where Tn is called the nth triangular number.
Examples: T5 = 5*6/2 = 15, and T6 = 6*7/2 = 21.
A number N is not a triangular number if it lies somewhere between Tn and T(n+1), for some n.
E.g. 20 is not a triangular number because it lies between T5 and T6.