Question

Triangular number

Answer 1

Triangular numbers are any of the series of numbers (1, 3, 6, 10, 15, etc.) obtained by continued summation of the natural numbers 1, 2, 3, 4, 5, etc.
Here the series is as follows
1=1
1+2=3
1+2+3=6
1+2+3+4=10
1+2+3+4+5=15
................

1,3,6,10,15,21,28....