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How do you find a missing column of a composed matrix from the columns of the second matrix?

The j-th column of the composed matrix A∘BA \circ B is the image under A of the j-th column of B. To find a missing column, identify the corresponding column in B, treat it as an input vector, and apply the transformation A to it. The result is the missing column.

Conditions

  • A is a linear transformation represented by a matrix.
  • Columns are read as vectors in the domain of A.

Reasoning, step by step

  1. Identify the index j of the missing column in A∘BA \circ B.
  2. Extract the j-th column from matrix B.
  3. Compute the product of matrix A and this column vector.
  4. The resulting vector is the j-th column of A∘BA \circ B.

Example

In the video, the middle column of A∘BA \circ B is missing. The middle column of B is [023]\begin{bmatrix} 0 \\ 2 \\ 3 \end{bmatrix}. Applying A to this vector gives [0−169]\begin{bmatrix} 0 \\ -16 \\ 9 \end{bmatrix}, which fills the blank spot.

Common misconceptions

  • Trying to transform the columns of A instead of B.
  • Assuming the missing column can be found by simple arithmetic on other columns without applying the transformation.

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