Continuous Proportions: Summing 1, 2, 4, 8, 16, 32, 64
Last minus first equals the sum of the intermediate terms; a non-proportional row and its differences
The folio compares two rows of numbers. The first, 1 2 4 8 16 32 64, stands in continuous (doubling) proportion, and Leonardo notes that subtracting the first from the last, 64 minus 1 equals 63, which is exactly the sum of the intermediate terms. The second row, 1 3 4 6 8 9 10 12 15 16, is not proportional; here last minus first equals the sum of the successive excesses between neighbouring numbers, which he tallies step by step to fourteen. A column in the outer margin lists those differences, 2 1 2 2 1 1 2 3 1, and their total, 14.
On this page
Geometric series 1 to 64 and the sum of its middle terms
Because the quantities 1 2 4 8 16 32 64 are in continuous proportion, taking the first from the last (one from sixty-four) leaves sixty-three, which equals the sum of the units found in the middle between one and sixty-four. The same holds for any other continuous proportion.
A non-proportional row and its column of differences summing to 14
For the row 1 3 4 6 8 9 10 12 15 16, which is in no proportion, last minus first equals all the excesses from one number to the next: two between one and three, one between three and four, and so on. Added together these give fourteen, listed in the margin as 2 1 2 2 1 1 2 3 1 and totalled 14.
