280 (number)

280 (two hundred [and] eighty) is the natural number after 279 and before 281.

Template:Numbers 200s
Ordinal	two hundred [and] eightieth
Cardinal	two hundred [and] eighty
Factorization	$2^{3}\cdot 5\cdot 7$
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

List of highways numbered 280

281 to 289

281

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

282

Two hundred [and] eighty-two 282 = 2·3·47, sphenic number

283

Two hundred [and] eighty-three 283 prime, twin prime with 281, strictly non-palindromic number

284

Two hundred [and] eighty-four 284 amicable number with 220; nontotient. 284 degrees Celsius is the flash point of paper.

285

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).

286

Two hundred [and] eighty-six 286 = 2·11·13, sphenic number, tetrahedral number, nontotient, also shorthand for the Intel 80286 microprocessor chip

287

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

288

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

289

Two hundred [and] eighty-nine 289 = 17², centered octagonal number, Friedman number since (8 + 9)^2 = 289. 289 = 1 + 1 + 4 + 27 + 256 = 0⁰ + 1¹ + 2² + 3³ + 4⁴