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Answers for “为什么行列式不是绝对值?”

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No, the determinant is a scalar value that can be positive, zero, or negative. It is explicitly not a matrix absolute value, despite the notation ∣A∣|A| sometimes being used.

Conditions: The object is a square matrix.; The notation ∣A∣|A| or det⁡(A)\det(A) is being interpreted.

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To find the area of the transformed region, multiply the original area by the absolute value of the determinant of the transformation matrix. The formula is: New Area=Old Area⋅∣det⁡(A)∣\text{New Area} = \text{Old Area} \cdot |\det(A)|.

Conditions: The transformation is linear and represented by a2×2a 2\times 2 matrix.; The original area is known.; Use ordinary Euclidean area in standard orthonormal coordinates.

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For a 2x2 matrix A=[abcd]A=\begin{bmatrix}a&b\\c&d\end{bmatrix}, the determinant is calculated as ad−bcad-bc. A square matrix is invertible exactly when its determinant is nonzero.

Conditions: The matrix must be a 2x2 square matrix for the specific formula.; The matrix must be square for the invertibility condition to apply.