The span of an infinite collection $\mathbf{v}_1, \mathbf{v}_2, \mathbf{v}_3, \ldots \in V$ of vector space elements is defined as the set of all finite linear combinations $\sum_{i=1}^n c_i \mathbf{v}_i$, where $n<\infty$ is finite but arbitrary.
(a) Prove that the span defines a subspace of the vector space $V$.
(b) What is the span of the monomials $1, x, x^2, x^3, \ldots$ ?