Let's assume each line is one toothpick.
In figure 1, there is a square made out of the 4 toothpicks.
In figure 2, there are four squares made. However, the 4 four toothpicks were already used, so we don't count them twice. That gives 4 (that were used) + 3 (the new ones on the left) + 3 (new ones on top) + 3 (new ones on right). We sum them up: 4 + 3 + 3 + 3 = 13.
In figure 3 there are nine squares made. We have the 13 squares made, plus 3 on the left column, 2 on the left center column, 3, on the center column, 2 on the right center, and 3 on the right. We total them up. 13 + 3 + 2 + 3 + 2+ 3 = 26.
Let's look at the pattern.
<u>Figure #</u> <u># Toothpick</u>s
1 4
2 13
3 26
From figure 1 to 2, we added 9. From 2 to 3 we added 13. If we draw figure 4, we would add (from left to right) 3, 2, 2, 3, 2, 2, 3. That would be 17 toothpicks added, and 39 total. Let's go back to the table.
<u>Figure #</u> <u># Toothpick</u>
1 4
2 13
3 26
4 39
Now let's look at the sequence happening. As we add more, we increase by 9, then 13, then 17. For each row, we add the same number as the previous row and then add 4. Drawing Figure 5 would be too much, but we can conclude based on the pattern that 21 squares would be added. We can take this sequence as far as we need to. We want the 10th figure or term, so we apply the pattern and keep going until we get there.
<u>Figure #</u> <u># Toothpick</u>
1 4
2 13 <--- add 9
3 26 <--- add 13, the same 9 and 4 more
4 39 <--- add 17, the same 13 and 4 more
5 60 <--- add 21, the same 17 and 4 more
6 85 <-- add 25, the same 21 and 4 more
7 114 <-- add 29, the same 25 and 4 more
8 147 <-- add 33
9 184 <---- add 37
10 225 <-- add 41
Thus the 10th figure has 225 toothpicks.
