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Exercise 1638Linear algebra › DeterminantsL1Added 3 Nov 2026

A numerical 4×4 determinant

#4x4 determinant#row reduction#computation

Statement

Compute
Δ=∣1234234134124123∣.\Delta = \begin{vmatrix} 1 & 2 & 3 & 4 \\ 2 & 3 & 4 & 1 \\ 3 & 4 & 1 & 2 \\ 4 & 1 & 2 & 3 \end{vmatrix}.

Hint

Add all columns to the first, factor, then subtract rows.

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Solution

Add columns. C1←C1+C2+C3+C4C_1\leftarrow C_1+C_2+C_3+C_4: each entry of column 1 becomes 1010. So
Δ=10∣1234134114121123∣.\Delta = 10\begin{vmatrix} 1 & 2 & 3 & 4 \\ 1 & 3 & 4 & 1 \\ 1 & 4 & 1 & 2 \\ 1 & 1 & 2 & 3 \end{vmatrix}.

Row operations. L2←L2−L1L_2\leftarrow L_2-L_1, L3←L3−L1L_3\leftarrow L_3-L_1, L4←L4−L1L_4\leftarrow L_4-L_1:
Δ=10∣1234011−302−2−20−1−1−1∣.\Delta = 10\begin{vmatrix} 1 & 2 & 3 & 4 \\ 0 & 1 & 1 & -3 \\ 0 & 2 & -2 & -2 \\ 0 & -1 & -1 & -1 \end{vmatrix}.

Expand along column 1.
Δ=10∣11−32−2−2−1−1−1∣.\Delta = 10\begin{vmatrix} 1 & 1 & -3 \\ 2 & -2 & -2 \\ -1 & -1 & -1 \end{vmatrix}.

Compute the 3×3. L2←L2−2L1L_2\leftarrow L_2-2L_1, L3←L3+L1L_3\leftarrow L_3+L_1:
=10∣11−30−4400−4∣=10⋅1⋅(−4)⋅(−4)=160.= 10\begin{vmatrix} 1 & 1 & -3 \\ 0 & -4 & 4 \\ 0 & 0 & -4 \end{vmatrix} = 10\cdot1\cdot(-4)\cdot(-4)=160.

Conclusion. Δ=160\Delta = 160.