Answer: If the dimension of V is n, then V has n elements.
Now, dim(U) + dim(W) = n, this means that the addition of the dimensions of U and W also has n elements.
and because U and W are subspaces of V, you know that every element on U and W is also an element of V.
If U ∩ W = ∅, means that there are no elements in common between U and W.
Because there are no elements in common, then Dim(U) + Dim(W) = dim(U ∪ W) = n
So U ∪ W has the same number of elements as V, and every element of W and U is also an element of V
this means that U ∪ W = V.
