The first four triangular numbers are 1, 3, 6 and 10, and the nth triangular number is given by the expression n(n+1)/2, so the 20th such number is 20 × 21 ÷ 2 = 210. ENIGMA is a triangular ...
Some results have been hidden because they may be inaccessible to you
Show inaccessible results
Feedback