Math Problem Statement

If matrices A, B and C are all non-zero matrices, are statements I and II true or false?

   Statement I:    AB = BC implies A = C.

   Statement II:   AB = AC implies B = C.

Explain, or give examples to justify your answer.

Solution

Let's analyze each statement one by one.

Statement I: AB = BC implies A = C.

This statement is false in general. To understand why, we can explore a counterexample.

Example:

Let: A=[10],B=[01],C=[10]A = \begin{bmatrix} 1 & 0 \end{bmatrix}, \quad B = \begin{bmatrix} 0 \\ 1 \end{bmatrix}, \quad C = \begin{bmatrix} 1 & 0 \end{bmatrix} Now, calculate ABAB and BCBC: AB=[10][01]=0AB = \begin{bmatrix} 1 & 0 \end{bmatrix} \begin{bmatrix} 0 \\ 1 \end{bmatrix} = 0 BC=[10][01]=0BC = \begin{bmatrix} 1 & 0 \end{bmatrix} \begin{bmatrix} 0 \\ 1 \end{bmatrix} = 0 So, AB=BCAB = BC, but ACA \neq C, because AA is a row matrix, and CC is also a row matrix, but they are the same in this case purely due to coincidence with the dimensions and entries. In general, the equality AB=BCAB = BC does not imply that A=CA = C.

Statement II: AB = AC implies B = C.

This statement is false in general as well. The key reason is that if AA is a non-invertible (singular) matrix, then it's possible for AB=ACAB = AC to hold without B=CB = C.

Example:

Let: A=[00],B=[11],C=[22]A = \begin{bmatrix} 0 & 0 \end{bmatrix}, \quad B = \begin{bmatrix} 1 \\ 1 \end{bmatrix}, \quad C = \begin{bmatrix} 2 \\ 2 \end{bmatrix} Now calculate ABAB and ACAC: AB=[00][11]=[0]AB = \begin{bmatrix} 0 & 0 \end{bmatrix} \begin{bmatrix} 1 \\ 1 \end{bmatrix} = \begin{bmatrix} 0 \end{bmatrix} AC=[00][22]=[0]AC = \begin{bmatrix} 0 & 0 \end{bmatrix} \begin{bmatrix} 2 \\ 2 \end{bmatrix} = \begin{bmatrix} 0 \end{bmatrix} So, AB=ACAB = AC, but BCB \neq C. This happens because AA is the zero matrix, and multiplying anything by the zero matrix results in the zero matrix, hence the equality holds but without BB and CC being equal.

Conclusion:

  • Statement I is false because AB=BCAB = BC does not imply A=CA = C.
  • Statement II is false because AB=ACAB = AC does not imply B=CB = C, especially when AA is singular (non-invertible).

Would you like more details or have any questions about these concepts? Here are some related questions to explore further:

  1. What is the condition for AB=ACAB = AC to imply B=CB = C?
  2. Under what conditions can AB=BCAB = BC imply A=CA = C?
  3. How does matrix invertibility affect the validity of these statements?
  4. What is the concept of matrix rank and how does it relate to these properties?
  5. What does it mean for a matrix to be singular or non-invertible?

Tip: A matrix is invertible if and only if its determinant is non-zero. Understanding this property can help determine conditions under which matrix equations like AB=ACAB = AC and AB=BCAB = BC hold.

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Math Problem Analysis

Mathematical Concepts

Matrix Multiplication
Linear Algebra
Matrix Properties

Formulas

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Theorems

Matrix equality
Non-invertible matrix behavior

Suitable Grade Level

Grades 11-12