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What is the final value of X required by the problem?

The final value of X is [100−930−401]\begin{bmatrix}1&0&0\\-9&3&0\\-4&0&1\end{bmatrix}. This is obtained by calculating the matrix product X2X1X_2X_1, where X1X_1 represents the first stage of row additions and X2X_2 represents the second stage of row scaling.

Conditions

  • X1=[100−310−401]X_1 = \begin{bmatrix}1&0&0\\-3&1&0\\-4&0&1\end{bmatrix}.
  • X2=[100030001]X_2 = \begin{bmatrix}1&0&0\\0&3&0\\0&0&1\end{bmatrix}.
  • The composition order is X=X2X1X = X_2X_1.

Reasoning, step by step

  1. Write down the matrices X2X_2 and X1X_1.
  2. Multiply X2X_2 by X1X_1 using standard matrix multiplication.
  3. Observe that left-multiplying by the diagonal matrix X2X_2 scales the second row of X1X_1 by 3.
  4. The first and third rows remain unchanged.
  5. The second row becomes 3×[−3,1,0]=[−9,3,0]3 \times [-3, 1, 0] = [-9, 3, 0].
  6. Combine the rows to form the final matrix X.

Example

The video calculates X=X2X1=[100030001][100−310−401]=[100−930−401]X = X_2 X_1 = \begin{bmatrix}1&0&0\\0&3&0\\0&0&1\end{bmatrix} \begin{bmatrix}1&0&0\\-3&1&0\\-4&0&1\end{bmatrix} = \begin{bmatrix}1&0&0\\-9&3&0\\-4&0&1\end{bmatrix}.

Common misconceptions

  • Reversing the multiplication order to X1X2X_1X_2.
  • Making arithmetic errors when multiplying the matrices.

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