The graph shows 3 figures formed by shaded and white triangles.
Pattern number |
1 |
2 |
3 |
... |
6 |
Number of shaded triangles |
0 |
2 |
5 |
... |
(i) |
Total number of triangles |
1 |
3 |
8 |
... |
(ii) |
- Complete the column for Figure 6. Give your answers for (i) and (ii) in numbers. (Eg 1, 2)
- What is the total number of triangles in Figure 13?
- How many shaded triangles are there in Figure 28?
(a)
Pattern for the number of shaded triangles
Figure 1: 0 =
0 =
1 - 1Figure 2: 0 + 2 =
2 =
1 + 2 - 1Figure 3: 0 + 2 + 3 =
5 =
1 + 2 + 3 - 1Formula:
Number of shaded triangles =
Sum of all numbers to Figure number - 1Number of shaded triangles in Figure 6
= (1 + 2 + 3 + ... + 6) - 1
= (6 x 7 ÷ 2) - 1
= 21 - 1
= 20 (i)
Pattern for the number of triangles from Figure 2 onwards.
Figure 2:
22 - 1 = 2 x 2 - 1 =
3 Figure 3:
32 - 1 = 3 x 3 - 1 =
8 Formula:
Number of triangles =
Figure number2 - 1 Total number of triangles in Figure 6
= 6
2 - 1
= 6 x 6 - 1
= 36 - 1
= 35 (ii)
(b)
Total number of triangles in Figure 13
= 13
2 - 1
= 13 x 13 - 1
= 169 - 1
= 168
(c)
Number of shaded triangles in Figure 28
= (1 + 2 + 3 + ... + 28) - 1
= (28 x 29 ÷ 2) - 1
= 406 - 1
= 405
Answer(s): (a) 20, 35; (b) 168; (c) 405