Subspace Preserved Under Addition of Elements? I'm trying to understand how to complete this proof about subspaces. I understand the basics about the definition of a subspace (i.e. the zero matrix must exist, and addition and multiplication must be closed within the subspace). But I'm confused as to how to show that the addition of two elements from completely different sets somehow are preserved under the same subspace. 
I'm pretty sure the zero vector exists because the zero vector is within C and D, but I'm unsure about the other two conditions. The complete problem is listed below. 
Problem: Let W be a vector space and C,D be two subspaces of W. Prove or disprove that { a + b | a $\in$ C, b $\in$ D} is also a subspace of W. 
Any help would be appreciated. 

You have to show (among other things) that $Z=\{{\,a+b:a\in C,b\in D\,\}}$ is closed under addition. So, let $x$ and $y$ be in $Z$; you have to show $x+y$ is in $Z$. So, what is $x$? Well, it's in $Z$, so $x=a+b$ for some $a$ in $C$ and some $b$ in $D$. What's $y$? Well, it's also in $Z$, so $y=r+s$ for some $r$ in $C$ and some $s$ in $D$. Now what's $x+y$? Can you take it from here? 
