Square Matrix with Duplicate Rows has Zero Determinant/Proof 2
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Theorem
If two rows of a square matrix over a commutative ring $\struct {R, +, \circ}$ are the same, then its determinant is zero.
Proof
Suppose that $\forall x \in R: x + x = 0 \implies x = 0$.
From Determinant with Rows Transposed, if you exchange two rows of a square matrix, the sign of its determinant changes.
If you exchange two identical rows of a square matrix, then the sign of its determinant changes from $D$, say, to $-D$.
But the matrix stays the same.
So $D = -D$ and so $D = 0$.
$\blacksquare$
Sources
- 1998: Richard Kaye and Robert Wilson: Linear Algebra ... (previous) ... (next): Part $\text I$: Matrices and vector spaces: $1$ Matrices: $1.6$ Determinant and trace: Proposition $1.9$