1. ## Subspace

Suppose $U_1$ and $U_2$ be subspaces of a vector space V. Define the sum of $U_1+U_2$ as follows:
$U_1+U_2$ = { $u_1+u_2$: $u_1$ is an element of $U_1$, $u_2$ is an element of $U_2$}

1. Prove that $U_1+U_2$ is a subspace of V.

2. Let W be a subspace of V such that $U_1$ subset or equal to W and $U_2$ subset or equal to W. Prove that $U_1+U_2$ subset or equal to W

Thanks

2. Originally Posted by apple12
Suppose $U_1$ and $U_2$ be subspaces of a vector space V. Define the sum of $U_1+U_2$ as follows:
$U_1+U_2$ = { $u_1+u_2$: $u_1$ is an element of $U_1$, $u_2$ is an element of $U_2$}

1. Prove that $U_1+U_2$ is a subspace of V.
clearly $(U_1 + U_2) \subseteq V$. it remains to show

(1) $U_1 + U_2$ is closed under addition and (2) closed under scalar multiplication

do you know what that means? can you show it?

2. Let W be a subspace of V such that $U_1$ subset or equal to W and $U_2$ subset or equal to W. Prove that $U_1+U_2$ subset or equal to W

Thanks
You need to show that $x \in (U_1 + U_2) \implies x \in W$

can you show this?