can some one solve this ?
Let W1 and W2 be subspaces of a vector space V. Prove that W1 union W2 is a subspace of V iff one of the subspaces contains the other.
The only if part is trivial. Here http://www.mathhelpforum.com/math-he...oup-proof.html, try to understand it. The proof to your problem is almost exactly the same.
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An alternate way to think of it is like this. $\displaystyle W_1,W_2$ are subspaces, thus, they are abelian groups under vector addition (definition of vector spaces). Thus, by the conditions of the problem $\displaystyle W_1
\cup W_2$ is a subspace and hence an abelian group. The link I gave you says, the union of two subgroups is a subgroup if and only if one is contained in the other. Now since $\displaystyle W_1
\cup W_2$ is an abelian group under addition of vectors follows that one of them is contained in the other. Thus, $\displaystyle W_1\subseteq W_2$ or $\displaystyle W_2\subseteq W_1$