A square number is a perfect square like 1, 4, 9, 16, 25, 36 ... A triangular number is the number of dots in an equilateral triangle: n(n+1)/2 = 1, 3, 6, 10, 15, 21, 28, 36 ... As you can see 1 and 36 are both square and triangular; in fact there are an infinite number of numbers that are both ...