Concept:
Here \(L(W_1\cup W_2)\) means the linear span of the set \(W_1\cup W_2\).
The sum of two subspaces is defined as
\[
W_1+W_2=\{w_1+w_2:w_1\in W_1,\ w_2\in W_2\}
\]
Step 1: Understand span of union.
The span of
\[
W_1\cup W_2
\]
contains all finite linear combinations of vectors from \(W_1\) and \(W_2\).
Therefore, every vector in the span can be written as
\[
w_1+w_2
\]
where
\[
w_1\in W_1,\qquad w_2\in W_2
\]
Step 2: Compare with sum of subspaces.
This is exactly the definition of
\[
W_1+W_2
\]
Therefore,
\[
L(W_1\cup W_2)=W_1+W_2
\]
Step 3: Final answer.
\[
\boxed{W_1+W_2}
\]