Exercise 3 Page 452

Let's draw the first three figures and count the number of balls in each of them.

We can turn this pattern into a sequence using the figure number n and the number of balls a_n. n:& 1, 2, 3 a_n:& 1, 3, 6 Examining the sequence, notice that, when compared to the previous figure, the number of balls increases by the figure number.

Therefore, to draw the fourth figure we need to add a fourth layer of balls.