
Re: Ariadne's thread
Posted:
Feb 28, 2012 3:07 AM


RMP 50 on the advanced level
Ahmes calculates the area of a circle whose diameter measures 9 khet = 900 royal cubits. By using his well known formula he obtains 8 by 8 khet = 64 square khet = 64 aroures or setat = 640,000 square cubits.
Advanced learners may try to solve a more demanding task: transforming the circle into a regular octagon of the same area using the following extended number pattern:
1 1 2 2
2 3 4 6
5 7 10 14
12 17 24 34
29 41 58 82
70 99 140 198
169 239 338 478
The squared side of a regular octagon and the area of the same octagon maintain a relation that can be approximated by means of the above numbers:
side x side 12 17 29 41 70 99 ... square cubits
area octagon 58 82 140 198 338 478 ... square cubits
The pi value may be chosen from the following sequence:
3/1 (plus 22/) 25/8 47/15 69/22 91/29 113/36 135/43 157/50 179/57 201/64 223/71 245/78 267/85 289/92 311/99 333/106 355/113 377/120 399/127
Two values contain the number 99: '99 of 478 and '99 of 311. Now the area of a regular octagon and the one of a circle may be defined like this:
side x side x '99 x 478, radius x radius x '99 x 311
The octagon and the circle have the same area, therefore:
side x side x '99 x 478 = radius x radius x '99 x 311
The diameter of the circle measures 9 khet or 900 royal cubits while the radius measures 450 royal cubits. Now we obtain:
side x side = 450 cubits x 450 cubits x 311 x '478
side x side = practically 131,752 square cubits
By consulting a table of square numbers you will find
362 x 362 = 131,044  708 less than 131,752
363 x 363 = 131,769  only 17 more than 131,752
364 x 364 = 132,496  744 more than 131,752
The number 363 is a good solution to our problem. Hence a circle of the diameter 9 khet and a regular octagon of the side length 363 royal cubits have practically the same area.
grid 770+1089+770 by 770+1089+770, unit '3 royal cubit

