280 (two hundred [and] eighty) is the natural number after 279 and before 281.
| Ordinal | two hundred [and] eightieth |
| Cardinal | two hundred [and] eighty |
| Factorization |  |
| Roman numeral | CCLXXX |
| Binary | 100011000 |
| Hexadecimal | 118 |
In mathematics
The denominator of the eighth harmonic number, 280 is an octagonal number.
There are 280 plane trees with ten nodes. As a consequence of this, 18 people around a round table can shake hands with each other in non-crossing ways, in 280 different ways (this includes rotations).
280 is a base 10 Harshad number.
In geography
See also the year 280.
281 to 289
Two hundred [and] eighty-one 281 prime, twin prime with 283, Sophie Germain prime, sum of the first fourteen primes, sum of seven consecutive primes (29 + 31 + 37 + 41 + 43 + 47 + 53), Chen prime, Eisenstein prime with no imaginary part, centered decagonal number
Two hundred [and] eighty-two 282 = 2·3·47, sphenic number
Two hundred [and] eighty-three 283 prime, twin prime with 281, strictly non-palindromic number
Two hundred [and] eighty-four 284 amicable number with 220; nontotient. 284 degrees Celsius is the flash point of paper.
Two hundred [and] eighty-five 285 = 3·5·19, sphenic number, square pyramidal number, Harshad number, also in Star Trek, the total number of Rules of Acquisition, repdigit in base 7 (555).
Two hundred [and] eighty-six 286 = 2·11·13, sphenic number, tetrahedral number, nontotient, also shorthand for the Intel 80286 microprocessor chip
Two hundred [and] eighty-seven 287 = 7·41, sum of three consecutive primes (89 + 97 + 101), sum of five consecutive primes (47 + 53 + 59 + 61 + 67), sum of nine consecutive primes (17 + 19 + 23 + 29 + 31 + 37 + 41 + 43 + 47), Kynea number, pentagonal number, also shorthand for the Intel math coprocessor to the 80286
Two hundred [and] eighty-eight 288 = 25·32, pentagonal pyramidal number, 4 superfactorial, untouchable number, Harshad number, self number, and is two-dozen sets of a dozen, thus being two gross, often told as a pun in maths classes. 288=32=11+22+33+44
Two hundred [and] eighty-nine 289 = 172, centered octagonal number, Friedman number since (8 + 9)^2 = 289
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