sand dollar. n. 1. Any of various thin circular echinoderms of the order Clypeasteroida, especially Echinarachnius parma, of coastal northern Atlantic and Pacific waters, having a covering of short movable spines.
One reason why linear algebra is very useful is that linear transforms of normal variables are normal. If x ~ N(a, A), then Bx+b ~ N(Ba+b, BABT). In a way, when everything is linear and normal, everything is nice and analytically tractable.
Only square matrices can have a determinant. One of the original uses of a determinant of a matrix had to do with solving a system of linear equations. Once the determinant of a matrix had been calculated, that number could be used to help quickly compute the solution to a system of equations.