280 (number)

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

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]}