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Answers for “如何计算变换后区域的面积?”

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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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The determinant scales the area of any measurable planar figure, not just squares. The absolute value of the determinant acts as a uniform area scaling factor for all regions under the linear transformation.

Conditions: The transformation is linear.; The figure is measurable and has finite area.; Use ordinary Euclidean area in standard orthonormal coordinates.

Understand why

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We multiply by 5 because the absolute value of the determinant of the transformation matrix A=[3112]A = \begin{bmatrix} 3 & 1 \\ 1 & 2 \end{bmatrix} is 5. The determinant represents the area scaling factor of the linear transformation.

Conditions: The transformation matrix is A=[3112]A = \begin{bmatrix} 3 & 1 \\ 1 & 2 \end{bmatrix}.; The original area is known.; Use ordinary Euclidean area in standard orthonormal coordinates.