Consider two subspaces $V$ and $W$ of $\mathbb{R}^{n} .$ Let $V+W$ be the set of all vectors in $\mathbb{R}^{n}$ of the form $\vec{v}+\vec{w},$ where $\vec{v}$ is in $V$ and $\vec{w}$ in $W$. Is $V+W$ necessarily a subspace of $\mathbb{R}^{n} ?$
If $V$ and $W$ are two distinct lines in $\mathbb{R}^{3},$ what is $V+W ?$ Draw a sketch.