# 90 (number)

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

 ← 89 90 91 →
Cardinalninety
Ordinal90th
(ninetieth)
Factorization2 × 32 × 5
Divisors1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
Greek numeralϞ´
Roman numeralXC
Binary10110102
Ternary101003
Quaternary11224
Quinary3305
Senary2306
Octal1328
Duodecimal7612
Vigesimal4A20
Base 362I36

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/.

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

## In mathematics

90 is:

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.

## 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