A crab is any of various crustaceans with four pairs of legs, a pair of pincers, a flattish shell, and a short, broad abdomen folded under its thorax. Several similar animals, such as the crab louse, or the hermit crab[?], are also called crabs.
A crab is any of various machines used to hoist heavy objects.
... formula is not ideal. See Loss of significance for details.
Derivation
The quadratic formula is derived by the method of completing the square[?]. ...