Expansions of (x+a)n as n = 0 to n
Each row is bounded by 1 on both sides
--- (The first row has only one item 1)First row is for n = 0
--- (The second row has two items 1 1)Second row is for n = 1
Any entry except the first and last entry in a row is the sum of two entries in the preceding row, one on the immediate left and the other on the immediate right.
So the third row (n = 2) is 1 2 1 (The second row elements are 1 1).