90 (number)

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 pronic number, as it is the product of 9 and 10. It is nontotient, and divisible by the sum of its base 10 digits, which makes it a Harshad number.
*It is the third unitary perfect number, since it is the sum of its unitary divisors excluding itself, and because it is equal to the sum of a subset of its divisors, it is also a semiperfect number.

*90 is a Stirling number of the second kind $S(n,k)$ from a $n$ of $6$ and a $k$ of $3$, as it is the number of ways of dividing a set of six objects into three non empty subsets.

*It is a Perrin number from a sum of 39 and 51, which average 45, the 9th triangular number or sum of the first nine non-zero positive integers.

The rhombic enneacontahedron is a zonohedron with a total of 90 rhombic faces: 60 broad rhombi akin to those in the rhombic dodecahedron with diagonals in $1:\sqrt2$ ratio, and another 30 slim rhombi with diagonals in $1:\varphi^$ golden ratio. The obtuse angle of the broad rhombic faces is also the dihedral angle of a regular icosahedron, with the obtuse angle in the faces of golden rhombi equal to the dihedral angle of a regular octahedron and the tetrahedral vertex-center-vertex angle, which is also the angle between Plateau borders: $109.471$°. The rhombic enneacontahedron is the zonohedrification of the regular dodecahedron, and it is the dual polyhedron to the rectified truncated icosahedron, a near-miss Johnson solid. On the other hand, the final stellation of the icosahedron has 90 edges. It also has 92 vertices like the rhombic enneacontahedron, when interpreted as a simple polyhedron.

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. Thus, an angle measuring 90 degrees is called a right angle.Friedman, Erich
What's Special About This 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 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 apart.
* The car number most associated with former NASCAR team owner Junie Donlavey
* The total number of minutes in an association football match.

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

{{Integers, zero

Integers