Raeann added a certain number of white squares round a grey square and then a certain number of grey squares round the white squares and she continued adding more grey and white squares round the diagram in each pattern as shown.
- How many grey squares did she need if the most outer layer had 13 squares on each side?
- How many squares are there in the Figure 20?
(a)
Formula:
Number of squares on each side = Figure number x 2 - 1
Figure number x 2 - 1 = 13
Figure number x 2 = 13 + 1 = 14
Figure number = 14 ÷ 2 = 7
Pattern for the total number of squares
Figure 1: 1 x 1 = 1 square
Figure 2: 3 x 3 = 9 squares
Figure 3: 5 x 5 = 25 squares
Formula:
Total number of squares = [(Figure number x 2) - 1]
2
Total number of squares at Figure 7
= ((7 x 2) - 1)
2 = 13 x 13
= 169
Figure 5 has 32 white squares (same as Figure 4).
Figure 6 has 40 white squares on its outermost layer.
Number of white squares at Figure 6
= 32 + 40
= 72
Number of white squares at Figure 7 = 72 (same as Figure 6)
Number of grey squares at Figure 7
= Total number of squares at Figure 7 - Number of white squares at Figure 7
= 169 - 72 = 97
(b)
Total number of squares at Figure 20
= [(20 x 2) - 1]
2 = 39 x 39 = 1521
Answer(s): (a) 97; (b) 1521