Linear Algebra : The Determinant

Study concepts, example questions & explanations for Linear Algebra

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Example Questions

Example Question #352 : Operations And Properties

True or false: Square matrix  is nilpotent if and only if .

Possible Answers:

False

True

Correct answer:

False

Explanation:

A square matrix  is nilpotent if, for some whole number ,  , the zero matrix of the same dimension as 

The determinant of the product of matrices is equal to the product of their determinants. It follows that if , so . Therefore, any nilpotent matrix must have determinant 0. 

However, not all matrices with determinant 0 are nilpotent, as is proved by counterexample. Let

This matrix is diagonal, as its only nonzero entry is alone its main diagonal. To raise this to a power , simply raise all of the diagonal elements to the power of , and preserve the off-diagonal zeroes. It follows that for all ,

Also, the determinant of  is 

Since a non-nilpotent matrix with determinant zero exists, the biconditonal is false.

Example Question #41 : The Determinant

, and  are each  real matrices. 

.

True or false: It follows that  is a singular matrix.  

Possible Answers:

False

True

Correct answer:

True

Explanation:

The determinant of the product of matrices is equal to the product of their determinants; thus,

.

A matrix has determinant 0 if and only if it is singular - that is, without an inverse. It therefore follows that  is singular.

Example Question #43 : The Determinant

Let , and  be three noncollinear points in Cartesian three-space. The equation of the plane through all three points is 

Give the equation of the plane that includes the points , and .

Possible Answers:

Correct answer:

Explanation:

If the three known points , and , and an unknown point  are coplanar, then 

The variable equation can be formed from this determinant equation.  

Using the first row, this can be rewritten as

,

where , the minor, is the determinant of the matrix formed by striking out Row 1 and Column 4:

Adding the upper-left to lower-right products and subtracting the upper-right to lower left products:

Set this equal to 0 to get the equation of the plane:

Example Question #42 : The Determinant

, and  are each  real matrices. 

.

True or false: It follows that  is a nonsingular matrix.  

Possible Answers:

True

False

Correct answer:

False

Explanation:

The statement can be proved false through counterexample.

Let

Each of the matrices has only zero elements on its off-diagonal elements, so each is a diagonal matrix. Consequently, the determinant of each matrix is the product of its diagonal elements.

 is a zero matrix, so  is an identity matrix, so ; and

, and  meet the conditions of the problem. Now, add the matrices by adding corresponding entries:

This matrix is also diagonal, so

A matrix has determinant 0 if and only if it is singular - that is, without an inverse. It therefore follows that  is singular. Thus, we have proved through counterexample that   need not be nonsingular.

Example Question #41 : The Determinant

, and  are each  real matrices. 

.

True or false: It follows that  is a singular matrix.  

Possible Answers:

True

False

Correct answer:

False

Explanation:

The statement can be proved false through counterexample.

Let

Each of the matrices has only zero elements on its off-diagonal elements, so each is a diagonal matrix. Consequently, the determinant of each matrix is the product of its diagonal elements.

 is a zero matrix, so  is an identity matrix, so ; and 

, and  meet the conditions of the problem. Now, add the matrices by adding corresponding entries:

This matrix is also diagonal, so

A matrix has a nonzero determinant if and only if it is nonsingular - that is, with an inverse. It therefore follows that  is nonsingular. Thus, we have proved through counterexample that   need not be singular.

Example Question #44 : The Determinant

Let , and  be three points in Cartesian three-space. These points are coplanar if and only if 

Three points are given: , and . Which point along the line of the equation 

is on the plane that includes these points? 

Possible Answers:

None of the other choices gives the correct response.

Correct answer:

Explanation:

The three points , and , and a point of the form  

 for some real  are on the same plane. Therefore, it follows that we must find  so that

Using the first row, this can be rewritten as

,

where , the minor, is the determinant of the matrix formed by striking out Row 1 and Column 4:

Adding the upper-left to lower-right products and subtracting the upper-right to lower left products:

Thus, 

,

and

et this equal to 0 and solve for :

The desired point is .

Example Question #47 : The Determinant

Let , and  be three points in Cartesian three-space. These points are coplanar if and only if 

Three points are given: , and . Which point along the line of the equation 

 

is on the plane that includes these points? 

Possible Answers:

None of the other choices gives the correct response.

Correct answer:

Explanation:

The three points , and , and a point of the form  for some real  are on the same plane. Therefore, it follows that we must find  so that

Using the first row, this can be rewritten as

,

where , the minor, is the determinant of the matrix formed by striking out Row 1 and Column 4:

Adding the upper-left to lower-right products and subtracting the upper-right to lower left products:

It follows that 

,

and

Set this equal to 0 to find :

The desired point is .

Example Question #354 : Operations And Properties

, and  are each  real matrices. 

.

True or false: It follows that  is a nonsingular matrix.  

Possible Answers:

False

True

Correct answer:

False

Explanation:

The statement can be proved false through counterexample.

Let

Each of the matrices has only zero elements on its off-diagonal elements, so each is a diagonal matrix. Consequently, the determinant of each matrix is the product of its diagonal elements.

 is a zero matrix, so  is an identity matrix, so ; and

, and  meet the conditions of the problem. Now, add the matrices by adding corresponding entries:

This matrix is also diagonal, so

A matrix has determinant 0 if and only if it is singular - that is, without an inverse. It therefore follows that  is singular. Thus, we have proved through counterexample that   need not be nonsingular.

Example Question #43 : The Determinant

 for some real such that  has an inverse.

Give in terms of .

Possible Answers:

None of the other choices gives the correct response.

Correct answer:

Explanation:

The inverse of a two-by-two matrix

is the matrix

.

is the determinant of the matrix, which is the product of the main diagonal elements minus the product of the other two:

The inverse is

.

Example Question #44 : The Determinant

Find .

Possible Answers:

Correct answer:

Explanation:

The inverse of a two-by-two matrix

is the matrix

.

is the determinant of the matrix, which is the product of the main diagonal elements minus the product of the other two:

Therefore,

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