Question 3. Let A = LU, where L is an invertible lower triangular matrix and U is upper triangular. Explain why the rst column of A is a multiple of the rst column of L. How is the second column of A related to the columns of L? 3 pts Question 4. Suppose A is a 5 X 3 matrix and that there exists a 3 X 5 matrix C such that CA = 13. (Here, 13 is the 3 X 3 identity matrix.) Suppose further that for some given b in R5, the equation AX = b has at least one solution. Show that this solution is unique. 3 pts