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
Ninety is a
pronic number as it is the
product of
9 and
10,[1] and along with
12 and
56, one of only a few pronic numbers whose digits in
decimal are also successive. 90 is divisible by the sum of its
base-ten digits, which makes it the thirty-second
Harshad number.[2]
90 can be expressed as the sum of distinct non-zero
squares in six ways, more than any smaller number (see image):[13]
90 is equal to the fifth sum of non-triangular numbers, respectively between the fifth and sixth triangular numbers,
15 and
21 (equivalently
16 +
17 ... +
20).[14] It is also twice
45, which is the ninth triangular number.
The members of the first
prime sextuplet (
7,
11,
13, 17,
19,
23) generate a
sum equal to 90, and the difference between respective members of the first and second prime sextuplets is also 90, where the second prime sextuplet is (
97,
101,
103,
107,
109,
113).[15][16] The last member of the second prime sextuplet, 113, is the 30th
prime number. Since prime sextuplets are formed from prime members of lower order
prime k-tuples, 90 is also a record maximal gap between various smaller pairs of prime k-tuples (which include
quintuplets,
quadruplets, and
triplets).[a]
The maximal number of pieces that can be obtained by cutting an
annulus with twelve cuts is 90, as is the number of 12-dimensional
polyominoes that are prime.[20]
This Witting configuration when reflected under the
finite space splits into points and planes, alongside lines.[22]
Whereas the rhombic enneacontahedron is the
zonohedrification of the regular dodecahedron,[23] a
honeycomb of Witting polytopes holds vertices
isomorphic to the lattice, whose symmetries can be traced back to the regular icosahedron via the
icosian ring.[24]
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
^90 is the record gap between the first pair of
prime quintuplets of the form (p, p+2, p+6, p+8, p+12) (
A201073), while 90 is a record between the second and third prime quintuplets that have the form (p, p+4, p+6, p+10, p+12) (
A201062). Regarding
prime quadruplets, 90 is the gap record between the second and third set of quadruplets (
A113404).
Prime triplets of the form (p, p+4, p+6) have a third record maximal gap of 90 between the second and ninth triplets (
A201596), and while there is no record gap of 90 for prime triplets of the form (p, p+2, p+6), the first and third record gaps are of
6 and 60 (
A201598), which are also unitary perfect numbers like 90 (
A002827).