280 (number)

This article covers the number 280. For the year, see 280.

← 279 280 281 →
← 200 210 220 230 240 250 260 270 280 290 → List of numbers Integers ← 0 100 200 300 400 500 600 700 800 900 →
Cardinal	two hundred eighty
Ordinal	280th (two hundred eightieth)
Factorization	2^3 × 5 × 7
Greek numeral	ΣΠ´
Roman numeral	CCLXXX
Binary	1000110002
Ternary	1011013
Senary	11446
Octal	4308
Duodecimal	1B412
Hexadecimal	11816

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

In mathematics

The denominator of the eighth harmonic number,^[1] 280 is an octagonal number.^[2] 280 is the smallest octagonal number that is a half of another 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). ^{[disputed – discuss]}

Integers from 281 to 289

281

Main article: 281 (number)

282

Main article: 282 (number)

283

Main article: 283 (number)

284

Main article: 284 (number)

285

Main article: 285 (number)

286

Main article: 286 (number)

287

Main article: 287 (number)

288

Main article: 288 (number)

289

Main article: 289 (number)

References

1. "Sloane's A002805 : Denominators of harmonic numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-28.
2. "Sloane's A000567 : Octagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-28.