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Answers for “如何计算两个线性变换 A 和 B 的复合矩阵?”

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To compute the matrix for the composition A∘BA \circ B, apply the transformation AA to each column of the matrix BB. The resulting vectors form the columns of the matrix A∘BA \circ B.

Conditions: A and B are both 3x3 matrices.; The operation is performed over the real numbers.; These are real linear maps with compatible input and output spaces, represented using standard column-vector coordinates.

Understand why

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In this context, A∘BA \circ B denotes the transformation obtained by applying B first and then applying A to the result. The notation follows the convention where the rightmost transformation acts first on the input vector.

Conditions: A and B are treated as transformations on the same space, here three-dimensional space.; The output of B must be an input acceptable to A.

Meet the concept

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The missing middle column is [0−169]\begin{bmatrix} 0 \\ -16 \\ 9 \end{bmatrix}. This is obtained by applying the transformation A to the middle column of B, which is [023]\begin{bmatrix} 0 \\ 2 \\ 3 \end{bmatrix}.

Conditions: A and B are the specific 3x3 matrices shown in the video.; The method of column-wise composition is used.

Understand why

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Rewriting a vector as a linear combination of standard basis vectors allows you to use the linearity of the transformation. Instead of computing the full matrix-vector product directly, you can apply the transformation to each basis vector separately (which corresponds to the columns of the matrix) and then combine the results using the original coefficients.

Conditions: The vectors are in R3R^3.; The coefficients are the coordinates of the vector relative to the standard basis.; A is linear: applying it preserves sums and scalar multiples.

Know when to use it

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When multiplying two matrices AA and BB to form ABAB, the transformation represented by the rightmost matrix BB acts first on the initial space, followed by the leftmost matrix AA acting on the already transformed result.

Conditions: Performing matrix multiplication ABAB; Matrices represent linear spatial transformations; Vectors are treated as column vectors

Understand why

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When computing the composition A∘BA \circ B, the matrix B represents the first transformation. Its columns are the images of the standard basis vectors under B.

Conditions: A and B are 3x3 matrices representing linear transformations.; The goal is to find the matrix representation of the composite transformation.