Fifteen (in Indian notation 15) is a natural number that precedes 16 and succeeds 14.
Summary
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- 1 Numerals
- 2 Arithmetic
- 3 Algebra
- 4 Other uses
- 5 Geometry
- 6 Sources
Numerals
- In binary: 1111
- In ternary 120
- Quaternary 33
- In quinario 30
- In octal 17
- In Mayan: three horizontal stripes
- In Romans XV
Arithmetic
- is an odd compound number
- is a multiple of 3 and 5
- is a number of the form 4j + 3, 6h + 3, 12m + 3
- is a number 7k + 1, 14p + 1
- is a difference of squares: 4 2– 1 2
Algebra
- the full 14th degree equation has exactly 15 terms
- the equation w 15-1 = 0 provides us with all the fifteenth roots of 1
- is a triangular number
Other uses
- A publication that appears every 15 days is called a fortnightly
- The payment to workers can be made, every 15 days, it is called “payment of quanciena”
- The numbers 1,2, …, 8,9 can be arranged in a square subdivided into 9 equal “squares”, so that in each row or column or on each diagonal, the sum of the respective numbers is always equal to fifteen.
- Reversing the figures of 15 and juxtaposing, two capicuas come out: 1551 and 5115
- A span of 15 years or three decades is called quindenium
Geometry
- A simple 15-sided polygon is called a pentadecagon
- The convex pentadecagon has 90 diagonals.
- The central angle of a regular pentadecagon is 24º and each interior angle measures 156º
