Chapter 3: Problem 2

What is wrong with the following argument? "If we add the first row of a determinant to the second row and the second row to the first row, then the first two rows of the determinant are identical, and the value of the determinant is zero. Therefore all determinants have the value zero."

Short Answer

Expert verified

The argument incorrectly generalizes the specific case of identical rows causing the determinant to be zero to all determinants.

Step by step solution

Identify the properties of determinants

Start by understanding the key properties of determinants. One important property is that if two rows of a matrix are identical, the determinant of the matrix is zero.

Analyze the row operations

review the given row operations: adding the first row to the second row and then adding the newly updated second row back to the first row.

Check the effect on rows

After the operation of adding the first row to the second row, the second row becomes a sum of the first and second rows. When the new second row is then added back to the first row, the first row becomes a sum of the original first and this new second row.

Evaluate the resulting matrix

Evaluate whether the resulting rows are truly identical; if they are not, the determinant may not necessarily be zero. If both rows are sums of each other, they might look different and not be identical.

Identify the logical fallacy

The argument assumes that because performing a certain operation on one specific determinant results in zero, this operation applies universally to all determinants, leading to their values being zero. This is a generalization error.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Matrix Properties

Understanding matrix properties is crucial in solving determinant problems. A matrix is a rectangular array of numbers arranged in rows and columns. The determinant of a matrix is a scalar value that is a function of the entries in a square matrix. Some important properties of determinants are:

If two rows (or two columns) of a matrix are identical, the determinant of the matrix is zero.
If a matrix has a row or column with all zeros, the determinant is zero.
The determinant of a diagonal or triangular matrix is the product of its diagonal elements.

These properties help determine whether operations performed on a matrix will affect its determinant value.

Row Operations

Row operations are procedures used to manipulate the rows of a matrix, often to simplify the matrix or make solving equations easier. Common row operations include:

Swapping two rows
Multiplying a row by a non-zero scalar
Adding or subtracting the multiple of one row to another row

When dealing with determinants, it's essential to note how these operations affect the determinant:

Swapping two rows changes the sign of the determinant.
Multiplying a row by a scalar multiplies the determinant by that scalar.
Adding a multiple of one row to another row does not change the determinant.

In the exercise given, understanding these properties helps us determine why the argument about row operations and identical rows might be flawed.

Logical Fallacy

A logical fallacy is an error in reasoning that renders an argument invalid. In this exercise, the fallacy lies in assuming that a specific operation on one determinant implies the same result for all determinants. This fallacy is known as overgeneralization or hasty generalization.
The argument states that adding rows to each other and obtaining zero means all determinants are zero. This assumption ignores the broader context and specific conditions under which determinants behave. It's crucial to check for logical rigor when making general claims.
Breaking down the fallacy:

The specific operation resulted in the rows appearing identical, making the determinant zero for that case.
This does not mean that performing the same operation on any matrix will always yield identical rows.
To conclude that all determinants are zero based on one example is incorrect and a misuse of inductive reasoning.

Zero Determinant

A zero determinant indicates that a matrix does not have an inverse, and thus, is singular. This can happen under several conditions:

Two rows or columns are identical.
All elements in a row or column are zero.
One row is a scalar multiple of another row.

In the given exercise, the operation of adding rows led to a zero value in the determinant for one specific case but doesn't universally imply zero for all matrices.
Understanding when a determinant is zero helps clarify why incorrect assumptions can lead to errors in logical reasoning. Using odd examples to generalize can lead to incorrect conclusions, highlighting the importance of context-specific analysis in mathematics.