All Linear Algebra Resources
Example Questions
Example Question #41 : The Determinant
True or false: Square matrix is nilpotent if and only if
.
True
False
False
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 #351 : Operations And Properties
,
, and
are each
real matrices.
;
;
.
True or false: It follows that is a singular matrix.
False
True
True
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 #354 : Operations And Properties
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
.
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 #355 : Operations And Properties
,
, and
are each
real matrices.
;
;
.
True or false: It follows that is a nonsingular matrix.
False
True
False
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 #42 : The Determinant
,
, and
are each
real matrices.
;
;
.
True or false: It follows that is a singular matrix.
False
True
False
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 #357 : Operations And Properties
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?
None of the other choices gives the correct response.
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?
None of the other choices gives the correct response.
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 #46 : The Determinant
,
, and
are each
real matrices.
;
;
.
True or false: It follows that is a nonsingular matrix.
False
True
False
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
for some real
such that
has an inverse.
Give in terms of
.
None of the other choices gives the correct response.
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 #432 : Linear Algebra
Find .
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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