The nth triangular number is n*(n+1)/2 for n = 1, 2, 3, ...
In order to figure out if a given number, K, is triangular or not, you need to be able to solve the quadratic
n2 + n - 2k = 0
If the solution is a positive integer then the number is a triangular number.
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78 42 are, as far as I can make out, two numbers. They do not make a triangle.78 42 are, as far as I can make out, two numbers. They do not make a triangle.78 42 are, as far as I can make out, two numbers. They do not make a triangle.78 42 are, as far as I can make out, two numbers. They do not make a triangle.
No
A triangle has only 1 face.
By Pascal
By making a triangle