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

• Because 90 is the sum of its unitary divisors (excluding itself), it is a unitary perfect number, and because it is equal to the sum of a subset of its divisors, it is a semiperfect number. 90 is a pronic number. But it is also a nontotient. It is a Perrin number, preceded in the sequence by 39, 51, 68.
• In normal space, the interior angles of a square 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.^[1] Thus, an angle measuring 90 degrees is called a right angle.^[2]
• 90 is divisible by the sum of its base 10 digits, thus it is a Harshad number.

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 of the North Pole and the South Pole are 90 degrees.
• Ninety is the address space for NOP (no operation) in computer memory

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. ↑ http://www.sparknotes.com/testprep/books/newsat/chapter20section4.rhtml
2. ↑ Friedman, Erich. What's Special About This Number?