This is a diagram of the first five rows of Pascalâ€™s triangle with a vertical dotted line passing down the middle of the triangle through the numbers 1, 2 and 6. This line is marked as the axis of symmetry because the numbers on one side of the line are reflected on the other side. On the left-hand side of the triangle, arrows point down the diagonal lines of the triangle, highlighting the number 1s on the edge of the triangle, the counting numbers, 1, 2, 3, … on the next diagonal line and the triangular numbers, 1, 3, 6, 10, … on the next diagonal. On the right-hand side, arrows point along the rows of the triangle to show the sum of the numbers in each row as follows. Row 2: 1 + 1 = 2 = 21. Row 3: 1 + 2 + 1 = 4 = 22. Row 4: 1 + 3 + 3 + 1 = 8 = 23.