A card from a free library — ask anything, no account, works offline. Every card carries its source.
Counting: how many ways to choose and to arrange
mathematics
From five things, there are 20 ordered pairs but only 10 unordered pairs — order is the whole difference between a permutation and a combination. Worked & sealed by the engine — P(5,2) = 5!/3! = 20 · C(5,2) = 10. [HOLDS; open the seal to re-check.]
source
card id
card_works_counting
address
UNPLACED
adjoining cards
- on the shelf of → The Works — sealed, worked demonstrations — a member of the the-works shelf in the keeping
- paves → The Floor of Discovery — one floor, and by its design the fear of God — A worked, engine-sealed demonstration — a paving-stone of the floor of reality.
- adjacent → The integers, examined: a prime, a divisor, a factorial — Neighbouring worked demonstrations in the same part of the volume.
- adjacent → Euler's identity, assembled from its parts — Neighbouring worked demonstrations in the same part of the volume.
Is this card incomplete? Tell the library — it will call out for more ↗