90 (number)

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

In English speech, the numbers 90 and 19 are often confused, as sounding 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/.

Look up ninety in Wiktionary, the free dictionary.

Interstate 90 is a freeway that runs from Washington to Massachusetts.

In mathematics

90 is:

• a unitary perfect number because it is the sum of its unitary divisors (excluding itself).^[1]
• a semiperfect number because it is equal to the sum of a subset of its divisors.^[2]
• a pronic number.^[3]
• a nontotient.^[4]
• a Perrin number, preceded in the sequence by 39, 51, 68.^[5]
• palindromic and a repdigit in bases 14 (66₁₄), 17 (55₁₇), 29 (33₂₉), and 44 (22₄₄).
• a Harshad number since 90 is divisible by the sum of its base 10 digits.^[6]

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.^[7] Thus, an angle measuring 90 degrees is called a right angle.^[8]

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) away in distance
• The car number most associated with former NASCAR team owner Junie Donlavey
• The number of minutes in a football (soccer) match.
• NFL: New York Jets Dennis Byrd's #90 is retired

In other fields

• +90 is the code for international direct dial phone calls to Turkey.
• 90 is the code for the French département Belfort.

References

1. ↑ "Sloane's A002827 : Unitary perfect numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29. 
2. ↑ "Sloane's A005835 : Pseudoperfect (or semiperfect) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29. 
3. ↑ "Sloane's A002378 : Oblong (or promic, pronic, or heteromecic) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29. 
4. ↑ "Sloane's A005277 : Nontotients". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29. 
5. ↑ "Sloane's A001608 : Perrin sequence". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29. 
6. ↑ "Sloane's A005349 : Niven (or Harshad) numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-29. 
7. ↑ http://www.sparknotes.com/testprep/books/newsat/chapter20section4.rhtml
8. ↑ Friedman, Erich. What's Special About This Number?