Are there restrictions on a and b? Maybe 0 < a < b or something like that?
An exercise in my book lists several sets of vectors, and I am to determine which sets constitutes a subspace in R^3.
According to my book, the set of all vectors on the form (a,b,0) is "not" a subspace in R^3, but I cannot understand why. The way a see it, it is closed under addition and scalar multiplication (it seems to equal the xy-plane). What am I missing? Thanks!