The figure is made of identical triangles.
- Complete the table for layers 8 and 15.
- If each small triangle has a height of 5 cm and a perpendicular base of 2 cm, find the area of all the triangles at the 25th layer.
Layer number |
Number of triangles |
1 |
1 |
2 |
3 |
3 |
5 |
4 |
7 |
... |
... |
8 |
(i) |
... |
... |
15 |
(ii) |
(a)
1st layer: 0 + 1 = 1 triangle
2nd layer: 1 + 2 = 3 triangles
3rd layer: 2 + 3 = 5 triangles
4th layer: 3 + 4 = 7 triangles
Formula:
Number of triangles in a layer = Pattern number + (Pattern number - 1)
Number of triangles for layer 8
= 8 + (8 - 1)
= 8 + 7
= 15
Number of triangles for layer 15
= 15 + (15 - 1)
= 15 + 14
= 29
(b)
Number of triangles in the 25th layer
= 25 + (25 - 1)
= 25 + 24
= 49
Area of one small triangle
=
12 x 5 x 2
= 5 cm
2Area of all the triangles at the 25th layer
= 5 x 49
= 245 cm
2 Answer(s): (a) 15, 29; (b) 245 cm
2