We have discussed elementary elimination matrices M_k in class. Prove the following two properties of elementary elimination matrices, which are very important for making LU factorization work efficiently in practice: M_k is nonsingular. Represent M_k^-1 explicitly and show that M_kM_k^-1 = M_k^-1M_k = I. The product of two elementary elimination matrices M_k and M_j with k notequalto j is essentially their "union"; and therefore they can be multiplied without any computational cost