Linear Algebra : Operations and Properties

Study concepts, example questions & explanations for Linear Algebra

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Example Questions

Example Question #191 : Linear Algebra

where  is a real number. 

Which expression is equal to the trace of ?

Possible Answers:

Correct answer:

Explanation:

The trace of a matrix is equal to the sum of the elements in its main diagonal - the elements going from upper left to lower right. Therefore,

Example Question #192 : Linear Algebra

Give the trace of .

Possible Answers:

Correct answer:

Explanation:

 and  are both diagonal matrices - their only nonzero elements are on their main diagonals, from upper left to lower right - so their product can be found by multiplying their corresponding diagonal elements:

The trace of a matrix is equal to the sum of the elements in its main diagonal, so the trace of  is

Example Question #21 : The Trace

 is a square diagonal matrix, the trace of which is equal to 0.

True or false:  must be singular.

Possible Answers:

False

True

Correct answer:

False

Explanation:

We can demonstrate this statement to be false with the counterexample

.

This matrix fits the definition of a diagonal matrix in that it has zeroes everywhere except on its main (upper-left-to-lower-right) diagonal. Also, its trace - the sum of the elements on its main diagonal - is

.

A matrix is singular - that is, without an inverse - if and only if its determinant is equal to 0. The determinant of a diagonal matrix is equal to the product of its diagonal elements, so

.

, a square diagonal matrix with trace 0, is not singular, proving the statement false through counterexample.

Example Question #121 : Operations And Properties

All skew-symmetric matrices have a trace of 

Possible Answers:

It depends on the skew-symmetric matrix given

Correct answer:

Explanation:

For any skew-symmetric matrix . Taking the trace of both sides we get

.

Example Question #22 : The Trace

Give the trace of .

Possible Answers:

Correct answer:

Explanation:

The trace of the matrix is the sum of the elements on its main (upper-left corner to lower-right corner) diagonal:

Example Question #23 : The Trace

 is a diagonal matrix with trace 0. 

True or false: It follows that  has trace 0.

Possible Answers:

True

False

Correct answer:

False

Explanation:

The statement is false, as can be demonstrated using the counterexample 

This matrix is diagonal, so it can be squared by squaring its individual elements:

The trace of a matrix is equal to the sum of the elements on its main diagonal, so 

but 

This gives us a matrix  with trace 0 whose square does not have trace 0. The statement is false.

Example Question #24 : The Trace

 and  are square matrices of the same dimension.

True or false: 

Possible Answers:

True

False

Correct answer:

True

Explanation:

The trace of a square matrix is equal to the sum of its diagonal elements - the elements whose row number and column number are equal. Therefore, if  and  are  matrices,

and 

By the addition rule for finite sums,

Also, addition of matrices is elementwise, so

Example Question #124 : Operations And Properties

The characteristic equation of a matrix  is 

Which statement must be true?

Possible Answers:

None of the other statements are necessarily true.

Correct answer:

Explanation:

The sum of the eigenvalues of a matrix is equal to the trace of the matrix. If  are the four (not necessarily distinct) eigenvalues of the matrix, then its characteristic equation is

When expanded, the coefficient of its  term is . It follows that the sum of the eigenvalues - and the trace of  - is .

Example Question #21 : The Trace

True or false: The trace of is 0.

Possible Answers:

True

False

Correct answer:

True

Explanation:

The trace of a matrix is the sum of the elements along its main diagonal, which are shown below:

The trace of can easily seen to be 0.

Example Question #128 : Operations And Properties

Give the trace of .

Possible Answers:

Correct answer:

Explanation:

The trace of a matrix is the sum of the elements along its main diagonal, which are shown below:

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