69. Continuation of Exercise 68 Let A=[aij] be an n×n matrix.
(a) Prove that if every element of a row or column of a matrix is multiplied by the real number c, then the determinant of the matrix is multiplied by c.
(b) Prove that if all the entries above the main diagonal (or all below it) of a matrix are zero, the determinant is the product of the elements on the main diagonal.