Why do we multiply the original area by 5 in this example?

Algebraically, for a matrix $\begin{bmatrix} a & b \\ c & d \end{bmatrix}$, the determinant is $ad - bc$. Geometrically, $ad$ captures the primary rectangular bounds, while subtracting $bc$ corrects for overlapping triangular regions created by off-diagonal shearing components.

Conditions: Matrix is 2x2; Entries are real numbers

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.

Geometrically, the determinant of $a 2\times 2$ matrix represents the factor by which the linear transformation scales areas. Specifically, it is the signed area of the parallelogram formed by the matrix's column vectors.

Conditions: The matrix is $2\times 2$.; The transformation is linear.; Use ordinary Euclidean area in standard orthonormal coordinates.

Applying matrix B and then matrix A composes the linear transformations. Since each transformation multiplies the area/volume by its respective determinant (including sign/orientation), the total scaling factor is the product $\det(A)\det(B)$.

Conditions: Matrices A and B are square and compatible for multiplication; Determinants are defined

In 3D, the determinant measures the volume scaling factor of a unit cube mapped to a parallelepiped. The sign indicates orientation: positive preserves the right-handed frame, while negative reverses it.

Conditions: Linear transformation in 3D space; Unit cube input