# 90 (number)

__: Natural number__

**Short description**
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Cardinal | ninety | |||

Ordinal | 90th (ninetieth) | |||

Factorization | 2 × 3^{2} × 5 | |||

Divisors | 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90 | |||

Greek numeral | Ϟ´ | |||

Roman numeral | XC | |||

Binary | 1011010_{2} | |||

Ternary | 10100_{3} | |||

Quaternary | 1122_{4} | |||

Quinary | 330_{5} | |||

Senary | 230_{6} | |||

Octal | 132_{8} | |||

Duodecimal | 76_{12} | |||

Hexadecimal | 5A_{16} | |||

Vigesimal | 4A_{20} | |||

Base 36 | 2I_{36} |

**90** (**ninety**) is the natural number preceded by 89 and followed by 91.

In the English language, the numbers 90 and 19 are often confused, as they sound very similar. When carefully enunciated, they differ in which syllable is stressed: 19 /naɪnˈtiːn/ vs 90 /ˈnaɪnti/. However, in dates such as 1999, and when contrasting numbers in the teens and when counting, such as 17, 18, 19, the stress shifts to the first syllable: 19 /ˈnaɪntiːn/.

## In mathematics

**90** is:

- a unitary perfect number because it is the sum of its unitary divisors (excluding itself): 1+2+5+9+10+18+45=90.
^{[1]} - a semiperfect number because it is equal to the sum of a subset of its divisors.
^{[2]} - a pronic number: 9 × 10.
^{[3]} - a nontotient.
^{[4]} - a Stirling number of the second kind, the number of ways of dividing a set of six objects into three non empty subsets.
^{[5]} - a Perrin number, preceded in the sequence by 39, 51, 68.
^{[6]} - a Harshad number since 90 is divisible by the sum of its base 10 digits.
^{[7]}

In normal space, the interior angles of a rectangle measure 90 degrees each. Also, in a right triangle, the angle opposing the hypotenuse measures 90 degrees, with the other two angles adding up to 90 for a total of 180 degrees.^{[8]} Thus, an angle measuring 90 degrees is called a right angle.^{[9]}

## In science

**Ninety** is:

- the atomic number of thorium, an actinide. As an atomic weight, 90 identifies an isotope of strontium, a by-product of nuclear reactions including fallout. It contaminates milk.
- the latitude in degrees of the North and the South geographical poles.

## In sports

- Nike Total 90 Apparel is a brand name of football apparel and football equipment from equipment bags to goalkeeper gloves
- Major League Baseball bases are 90 feet (27 m) apart.
- The car number most associated with former NASCAR team owner Junie Donlavey

## In other fields

- +90 is the code for international direct dial phone calls to Turkey.
- 90 is the code for the French département Belfort.
- Interstate 90 is a major east-west controlled access highway that spans the continental
*United States*for 3,020 miles from Seattle to Boston.

## References

- ↑ "Sloane's A002827 : Unitary perfect numbers". OEIS Foundation. https://oeis.org/A002827.
- ↑ "Sloane's A005835 : Pseudoperfect (or semiperfect) numbers". OEIS Foundation. https://oeis.org/A005835.
- ↑ "Sloane's A002378 : Oblong (or promic, pronic, or heteromecic) numbers". OEIS Foundation. https://oeis.org/A002378.
- ↑ "Sloane's A005277 : Nontotients". OEIS Foundation. https://oeis.org/A005277.
- ↑ "Sloane's A008277 :Triangle of Stirling numbers of the second kind". OEIS Foundation. https://oeis.org/A008277.
- ↑ "Sloane's A001608 : Perrin sequence". OEIS Foundation. https://oeis.org/A001608.
- ↑ "Sloane's A005349 : Niven (or Harshad) numbers". OEIS Foundation. https://oeis.org/A005349.
- ↑ http://www.sparknotes.com/testprep/books/newsat/chapter20section4.rhtml
- ↑ Friedman, Erich. What's Special About This Number?

Original source: https://en.wikipedia.org/wiki/90 (number).
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