Linear Algebra : Linear Equations

Study concepts, example questions & explanations for Linear Algebra

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Example Questions

Example Question #31 : Reduced Row Echelon Form And Row Operations

True or false:

is an example of a matrix in row-echelon form.

Possible Answers:

False

True

Correct answer:

True

Explanation:

A matrix is in row-echelon form if and only if it fits three conditions:

1) Any rows comprising only zeroes are at the bottom. 

2) Any leading nonzero entries are 1's..

3) Each leading 1 is to the right of the one immediately above.

It can be seen that this matrix fits all three conditions, and is therefore in row-echelon form.

Example Question #31 : Reduced Row Echelon Form And Row Operations

True or false:  is an example of a matrix in row-echelon form.

Possible Answers:

True

False

Correct answer:

False

Explanation:

A matrix is in row-echelon form if and only if it fits three conditions:

1) Any rows comprising only zeroes are at the bottom. 

2) Any leading nonzero entries are 1's..

3) Each leading 1 is to the right of the one immediately above.

All four rows have leading nonzero entries, but none of them are 1's. The matrix violates the conditions of a matrix in row-echelon form.

Example Question #31 : Reduced Row Echelon Form And Row Operations

Is the following matrix in reduced row echelon form? Why or why not.

Possible Answers:

Yes, the matrix is in reduced row echelon form

No, the matrix has nonzero elements

No, the matrix does not have all zero rows

No, the matrix has non zero lead entries

Correct answer:

Yes, the matrix is in reduced row echelon form

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

This matrix meets all four of these criteria.

Example Question #31 : Reduced Row Echelon Form And Row Operations

Is the following matrix in reduced row echelon form? Why or why not.

Possible Answers:

Yes, the matrix is in reduced row echelon form

No, the matrix does not have an all zero row

No, the matrix has nonzero elements

No, the leading entry in the first row is not 

Correct answer:

No, the leading entry in the first row is not 

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

For this matrix to be in reduced row echelon form, the leading entry in the first row would need to be , not .

Example Question #31 : Reduced Row Echelon Form And Row Operations

Is the following matrix in reduced row echelon form? Why or why not.

Possible Answers:

No, the matrix has nonzero terms

No, there is a nonzero row below an all zero row.

Yes, the matrix is in reduced row echelon form

Yes, the leading entries are all 

Correct answer:

No, there is a nonzero row below an all zero row.

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

For the matrix to be in reduced row echelon form, the  and  row would beed to be switched.

Example Question #32 : Reduced Row Echelon Form And Row Operations

Is the following matrix in reduced row echelon form? Why or why not.

Possible Answers:

No, the matrix has nonzero terms

No, there are no nonzero rows

No, there is a nonzero entry in the same column as a leading entry

Yes, the matrix is in reduced row echelon form

Correct answer:

No, there is a nonzero entry in the same column as a leading entry

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

For the matrix to be in reduced row echelon form, the  in the first row would need to be transformed into a  using row operations.

Example Question #31 : Reduced Row Echelon Form And Row Operations

Used reduced row echelon form to solve the following system of equations

Possible Answers:

a

Correct answer:

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

The first step in solving this system of linear equations is to represent this system as an augmented matrix.

 

We will use row operations to transform this matrix. There are three kinds of row operations that can be performed.  Rows can be switched, a row can be multiplied by a constant and rows can be linearly combined ( where  and  are constants).

The first number in the  row is , so no further calculation is needed. 

We will make the first number in the  row  using the row operation 

We now make the leading entry of the  row  using the row operation 

Next we will make the first number in the  row  using the row operation 

That operation also made the  number in the  row .

We now make the leading entry of the  row  using the row operation 

 

The matrix is now in echelon form.

Next we change the  term in the  row to  using the row operation 

Next we change the  term in the  row to  using the row operation 

 

Last we change the  term in the  row to  using the row operation 

The matrix is now in reduced row echelon form and the last column is the solution to the system of equations.

 

Example Question #31 : Reduced Row Echelon Form And Row Operations

Used reduced row echelon form to solve the following system of equations

 

Possible Answers:

Correct answer:

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

The first step in solving this system of linear equations is to represent this system as an augmented matrix

 

We will use row operations to transform this matrix. There are three kinds of row operations that can be performed.  Rows can be switched, a row can be multiplied by a constant and rows can be linearly combined ( where  and  are constants).

Since the leading entry in the first row is , no further action is needed.

 

Next we will make the first number in the  row  using the row operation 

We now make the leading entry of the  row  using the row operation 

Next we will make the first number in the  row  using the row operation 

The matrix is now in echelon form.

Next we change the  term in the  row to  using the row operation 

Next we change the  term in the  row to  using the row operation 

Last we change the  term in the  row to  using the row operation 

The matrix is now in reduced row echelon form and the last column is the solution to the system of equations.

 

Example Question #31 : Reduced Row Echelon Form And Row Operations

Used reduced row echelon form to solve the following system of equations

Possible Answers:

Correct answer:

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

The first step in solving this system of linear equations is to represent this system as an augmented matrix.

 

We will use row operations to transform this matrix. There are three kinds of row operations that can be performed.  Rows can be switched, a row can be multiplied by a constant and rows can be linearly combined ( where  and  are constants).

First we will make the first number in the  row  using the row operation 

 

Next we will make the first number in the  row  using the row operation 

We now make the leading entry of the  row  using the row operation 

Next we will make the first number in the  row  using the row operation 

Next we will make the  number in the  row  using the row operation 

We now make the leading entry of the  row  using the row operation 

 

The matrix is now in echelon form.

Next we change the  term in the  row to  using the row operation 

Last we change the  term in the  row to  using the row operation 

The matrix is now in reduced row echelon form and the last column is the solution to the system of equations.

 

Example Question #31 : Reduced Row Echelon Form And Row Operations

Used reduced row echelon form to solve the following system of equations

Possible Answers:

Correct answer:

Explanation:

A matrix is in reduced row echelon form if

  • all nonzero rows are above any all zero rows
  • the left most nonzero entry in each row (the leading entry) is .
  • the leading entry is in a column to the right of the leading entry in the column above it
  • all entries in a column below the leading entry are zero

The first step in solving this system of linear equations is to represent this system as an augmented matrix.

 

We will use row operations to transform this matrix. There are three kinds of row operations that can be performed.  Rows can be switched, a row can be multiplied by a constant and rows can be linearly combined ( where  and  are constants).

The leading entry in the  row needs to be , so we use the row operation  to change it.

We will make the first number in the  row  using the row operation 

We now make the leading entry of the  row  using the row operation 

Next we will make the first number in the  row  using the row operation 

Next we make the  number in the  row  using the row operation 

We now make the leading entry of the  row  using the row operation 

 

The matrix is now in echelon form.

Next we change the  term in the  row to  using the row operation 

Next we change the  term in the  row to  using the row operation 

 

Last we change the  term in the  row to  using the row operation 

The matrix is now in reduced row echelon form and the last column is the solution to the system of equations.

 

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