D D 1 D2 D3 are four determinants, the first is the coefficient determinant, and the last few are replaced by constant terms.
The third-order determinant is obtained by changing the equation coefficients of the corresponding unknowns A, B and C.
1 1 1 0 1 1 1 0 1 1 1 0
d = 4 2 1d 1 = 3 2 1 D2 = 4 3 1 D3 = 4 2 3
9 -3 1 28 -3 1 9 28 1 9 -3 28