(a) (3 marks) Let A be a 4x4 matrix with det(A) = 0. If 1,3 and 5 are eigenvalues of A,give one reason why A is diagonalizable. (b) (3 marks) If A is a 4 x 4 matrix with three distinct eigenvalues 11, 12, 13 (all real numbers) and if the dimension of the eigenspace E, is 2, prove that A is diagonalizable.