Law: The nth triangle number is the sum of the first n natural numbers.

General formula: n × (n+ 1)/2.

20 16*2=n × (n + 1)

n=63

Then 20 16 is the 63rd triangle number.

Extended data

Special case of triangle number

1,55, 5050, 500500, 50005000 ... are all triangular numbers.

2. The number of triangles (66) is 1 1, Triangles with the quantity of 1 1 1 (6177111triangle (61) The quantities are but11,1 1 1 and1/65438.

3. There is also a rule for the number of triangles, that is, if all polygons are drawn neatly from left to right in the table, you will find that the number interval of each column is the same, which is the number of triangles in the previous column.