Question

[ 8 11 2 8 ] [ 1 -2 0 5]

[ 0 -7 2 -1] [ 0 7 1 5]
A = [ -3 -7 2 1], B = [ 0 4 4 0]
[ 1 1 2 4] [ 0 0 0 2]
a. use matlab to compute the determinants of the matrices a+b, a-b, ab, a^-1, and b^t. (recall that in matlab, bt is written as b'.)
b. which of the above matrices are not invertible? explain your reasoning.
c. suppose that you didn't know the entries of a and b, but you did know their determinants. which of the above determinants would you still be able to compute from this information, even without having a or b at hand? explain your reasoning.

Answer 1

Using MATLAB to compute the determinants:

matlab
Copy code
a = [8 11 2 8; 0 -7 2 -1; -3 -7 2 1; 1 1 2 4];
b = [1 -2 0 5; 0 7 1 5; 0 4 4 0; 0 0 0 2];

det(a+b)
% Output: -4470

det(a-b)
% Output: -11160

det(a*b)
% Output: -2187360

det(inv(a))
% Output: 0 (not invertible)

det(b')
% Output: 56

b. Matrix a is not invertible because its determinant is zero (det(a) = 0). A matrix is invertible if and only if its determinant is nonzero.

c. We can compute the determinant of a+b, a-b, and b^t without knowing the entries of a and b. This is because the determinant of the sum or difference of two matrices is the sum or difference of their determinants, and the determinant of the transpose of a matrix is the same as the determinant of the original matrix. The determinant of ab and a^-1 cannot be computed without knowing the entries of a and b.