Not necessarily. [tex]\mathbf u[/tex] and [tex]\mathbf v[/tex] may be linearly dependent, so that their span forms a subspace of [tex]\mathbb R^2[/tex] that does not contain every vector in [tex]\mathbb R^2[/tex].
For example, we could have [tex]\mathbf u=(0,1)[/tex] and [tex]\mathbf v=(0,-1)[/tex]. Any vector [tex]\mathbf w[/tex] of the form [tex](r,0)[/tex], where [tex]r\neq0[/tex], is impossible to obtain as a linear combination of these [tex]\mathbf u[/tex] and [tex]\mathbf v[/tex], since
[tex]c_1\mathbf u+c_2\mathbf v=(0,c_1)+(0,-c_2)=(0,c_1-c_2)\neq(r,0)[/tex]
unless [tex]r=0[/tex] and [tex]c_1=c_2[/tex].
