45 (forty-five) is the natural number following 44 and preceding 46.The number 45 is an odd composite number (3²×5), recognized as the 9th triangular number and a Kaprekar number.
In mathematics
thumb|200px|45 as the difference of two nonzero squares (in orange)
45 is a composite number and a Størmer number.
Forty-five is conjectured from Ramsey number <math>R(5, 5)</math>.
<math>\phi(45)=\phi(\sigma(45))</math>
Forty-five degrees is half of a right angle (90°).
Forty-five is the sum of all single-digit decimal digits: <math>0+1+2+3+4+5+6+7+8+9=45</math>. It is, equivalently, the ninth triangle number. Forty-five is also the fourth hexagonal number and the second hexadecagonal number, or 16-gonal number.
In music
thumb|99px|45 [[revolutions per minute|rpm gramophone record]]
- A type of gramophone record classified by its rotational speed of 45 revolutions per minute (rpm)
In other fields
- Guns or ammunition of .45 caliber. In the United States, "45" is often a reference to one of two specific .45 caliber cartridges—the .45 Colt or the .45 ACP.
- In years of marriage, the sapphire wedding anniversary.
- Issue 45 of The North Briton was thought to be seditious but its publisher, John Wilkes, was celebrated as a champion of liberty. The number 45 was used as a symbol of support for him. Banquets were held with a theme of 45 while many items were produced showing the number or featuring it in some way. For example, a wig was produced with 45 curls.