Question

How do you get the rule of 1,3,6,10,15,21,28

Answer 1

It's simply the triangular numbers: 1=1 1+2=3 1+2+3=6 1+2+3+4=10 and so on. You could use the formula n(n+1)/2 to determine the nth triangular number (ie the sum of the first n numbers)

Answer 2

Well, the sequence is basically generated by a triangle order.
You need to add a new row of dots beneath the other one every time you've completed one triangle