An m times n upper triangular matrix is one whose entries below the main diagonal are zeros, as is shown in the matrix to the right. when is a square upper triangular matrix invertible? justify your answer.

A square matrix is invertible when its determinant is non-zero. An upper-triangular square matrix will have a non-zero determinant if there are no zeros on the diagonal.

The determinant is the sum of the values in a row or column multiplied by the determinant of the corresponding cofactor matrix. When all matrix entries below the diagonal are zero, the determinant becomes the product of diagonal terms.

what is the original number? you forgot to add it.