81

Thân thiện

A child counts 81 colorful marbles on the floor.

Definition

1. Adjective:
   • Being one more than eighty: The number 81 is the natural number that follows 80 and precedes 82. It is represented by the numerals '81' or the Roman numeral 'LXXXI'.

Usage

• The word '81' is a cardinal number used for counting, ordering, and labeling. It functions primarily as an adjective when it modifies a noun to indicate quantity or position.
  • The book has 81 pages. (It modifies the noun "pages" to specify the quantity.)
  • She finished in 81st place. (Here, the ordinal form "81st" indicates position in a sequence.)

Examples

• Adjective:
  • The temperature reached 81 degrees Fahrenheit today.
  • He scored 81 points in the basketball game.
  • The project will take 81 days to complete.

Advanced Usage

• As a numeral in mathematics: 81 is a perfect square (9²), a perfect fourth power (3⁴), and the sum of the first nine prime numbers (2+3+5+7+11+13+17+19+23).
  • The square root of 81 is 9.
• In cultural or historical contexts: The number may be used symbolically or refer to specific years, ages, or identifiers.
  • He was born in '81. (Referring to the year 1981 or 1881, depending on context.)

Variants and Related Words

• Eighty-one: The word form of the number.
• 81st (eighty-first): The ordinal number form.
• LXXXI: The Roman numeral representation.

Synonyms

• Eighty-one: The literal synonym in word form.
• Four score and one: An archaic phrasing (from "four score" meaning eighty).

Related Phrases

• Chapter 81: Used to label a specific section in a book or document.
• Route 81 / Highway 81: Used in the names of roads or highways.
• 81 years old: A phrase describing age.

Học thuật

Adjective

1. being one more than eighty

Từ đồng nghĩa

• eighty-one
• lxxxi