B: (0, a)
C should have the same y value as B
D: (a,0)
C should have the same x value as D
So point C is (a,a)
Since point A is on the origin, its point is (0,0)
You use the slope formula and plug in point A and C:
m = 1
So the value that belongs in the green box is 1
From the recursive rule
it follows that
and so on. Notice how the subscript on a on the right side and the coefficient multiplied by 13 add up to n (n - 2 + 2 = n; n - 3 + 3 = n; and so on). If we continue the pattern, we'll end up with
so that the explicit rule for the n-th term in the sequence is