# 90 (number): Wikis

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# Encyclopedia

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

90
Cardinal ninety
Ordinal 90th
(ninetieth)
Numeral system 90
Factorization $2 \cdot 3^2 \cdot 5$
Divisors 1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90
Roman numeral XC
Binary 10110102
Octal 1328
Duodecimal 7612

## 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. 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.

Ninety is:

## References

1. ^ Various fanboys and fansites use this nickname. See for example Ninty.net and the South African Nintendo Community.