9. (5%) Let S1 and S2 be subspaces ofRn and S = S1+ S2, the sum of the two subspaces. Let D1 = , the set of elements in S but not in S2. Then, D1 is also a subspace, and its dimension dim(D1) = dim(S1).

8. (5%) Consider a system of linear equations At = b, where A is a 2 x 3 matrix, I is a 3 x 1 column vector, and b is a 2 X 1 column vector. LetIn other words, all entries of A are non-zero. Then, it is impossible to uniquely determine t1 from the above system of linear equations.