The 90 (number) reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

90 (number)

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

List of numbers — Integers << 90 91 92 93 94 95 96 97 98 99 >>
Cardinal	ninety
Ordinal	90th (ninetieth)
Factorization
Divisors	2, 3, 5, 6, 9, 15, 18, 30, 45
Roman numeral	XC
Binary	1011010
Hexadecimal	5A

In mathematics

Ninety is the sums of the squares of the integers 2 to 6, as well as a highly composite number. Because 90 is the sum of its unitary divisors (excluding itself), it is a unitary perfect number. 90 is a heteromecic number. And because 90 is divisible by the sum of its digits in base 10, it is a Harshad number. But it is also a nontotient.

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.

In other fields

Ninety is:

• The atomic number of thorium, an actinide.
• The designation of American Interstate 90, a freeway that runs from Washington to Massachusetts.
• The code for international direct dial phone calls to Turkey.
• The year AD 90, 90 BC, or 1990.

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