Introduction to matrices
Introduction to Matrices
Before we start solving multiple equations, I have to introduce the matrix calculations.
Tables can be represented by what we call matrices.
A matrix is a type of rectangular array of numbers.
The dimensions of a matrix are the number of rows and columns. A matrix having 3 rows and 5 columns is called a matrix with dimensions of spelled “3 by 5”.
Or
Or another valid notation:
Or even
All these forms are valid and can be used. We’ll use the first one for this text.
The numbers in a matrix are called . Each element is accessed using an address.
The element is named by the intersection of the row and the column as seen in normal spreadsheets.
If we read the element located at the third row and second column, we can that we have 15.
We can say
Comparing matrices:
Two matrices are equal when they have same dimensions with equivalent corresponding elements. Operations on matrices: Addition and subtraction: When two matrices have the same dimensions, we can add them or subtract them by merely adding or subtracting the corresponding elements.
If we have the following matrices:
We can add or subtract because they have the same dimensions
For the subtraction:
Scalar multiplication:
To multiply a matrix by a number, multiply all elements by the number:
-If we multiply a by a real number , all elements have to be multiplied by that number.
If , the scalar multiplication will give us the of the original matrix.
The matrix is the of matrix .
Properties:
The following algebraic properties we learned apply to matrix addition:
-Commutativity:
-Additive identity : The matrix with only for all elements is the identity matrix for our addition.
-Associativity
-Finally, the additive inverse has all elements opposite to the corresponding elements of
Matrix multiplication
The golden rule in matrix multiplication is to have the following condition:
The number of columns of the first matrix must be equal to the number of rows of the second matrix.
The resulting matrix has the number of rows equal to the one of the first matrix and the number of columns equal to the one of the second matrix.
If and , for the multiplication to be possible we must have
After the multiplication,: will be of dimensions
Process: To find an element of located at row and column of , we have to take the sum of the products of corresponding elements in row of and column of
Example:
In this case :
is a matrix.
is a matrix
The matrix product = is a matrix.
Another example
is a matrix.
is a matrix.
The matrix product = is a matrix.
Now for bigger matrices
is a matrix.
is a matrix.
The matrix product = is a matrix.
Finally:
In an matrix, each element can be represented by where is the row of the element and is the column of the element.
What we have learned so far:
If and :
Matrix iff for every and ,
Matrix iff for every and ,
The noted of matrix is the matrix
The product of a real number and a matrix is equal to
When a multiplication is possible, the matrix product of matrices and has elements
Important matrices
Matlab relies heavily on matrices manipulation.
It is important that we know some particular matrices.
Square matrix:
A square matrix is a matrix that has the same number of rows and columns.
If is a matrix, we can say that is a .
In this case, is a matrix of order .
The main diagonal elements of a square matrix of order are
The Identity matrix
This is a square matrix noted is a matrix of order with each main diagonal element equal to and all the other elements .
Examples
We have the following interesting fact:
Inverse of a matrix
The inverse of matrix of order noted , called is such:
It is important to note that some matrices have no inverse or are not invertible.
We’ll see it later. Finding inverse without using may be tedious.
We’ll see it on the videos that go with this refresher.
When we solve the equation , we can find the elements of
The following example shows what I mean:
And
We can say
But since is of order 2.
We have
We find:
Finally:
Matrix Transpose:
The transpose of a Matrix noted is a new matrix whose columns are the rows of the original and making its rows, the columns of the original.
If we have:
We can notice that:
is a matrix and
is a matrix.
We can say:
If
A small rule:
Upper and lower triangular matrices:
The diagonal of a square matrix may show two type of matrices:
-When all elements above the diagonal are zeros, we get the lower-triangular matrix.
-On the other hand, if the lower part consists of zeros, the matrix is upper-triangular
Symmetric matrix:
A symmetric matrix is a square matrix equal to its transpose.
If is symmetric, we have =.
Diagonal matrix:
A diagonal matrix is a symmetric matrix with all elements zeros but the ones on the diagonal.
Determinants: Rule of Sarrus
For 3 x 3 matrices, there is a rule called Rule of Sarrus or Sarrus’ Rule used to simplify the complicated calculations of the determinants. With this method, there is no need to calculate cofactors. There are other methods used for the 4 x 4 and above matrices. However, in this text, we will just limit our calculations to the 3 x 3 dimensions matrices.
How does it work?
We add the first column as the fourth column and we add the second column as the fifth column.
If we have a matrix:
The Determinant
Now let’s add the two first columns:
Now we take the sum of the downward diagonals of 3 elements each and we subtract the sum of the upward diagonals:
Solve using Sarrus’ Rule:
Now for
Now for
Now for
Thus, the solution is
Project Bonus: Find a number xyzuw
A number of 5 digits can be written xyzuw.
The sum of the digits is 22.
If the number is rewritten xywuz, the value increases by 297.
The digit x added to digit u and to 2 times the digits y and w, then subtracted 3 times the digit z equal 21.
Two times the digit x added to digit z then subtracted 3 times digit y and 4 times digit w equal -34.
Two times the digit u subtracted digit w and 3 times digit z equal -1.
Find the number xyzuw.
Solving by substitution:
From we can write:
From :
Replacing by its value of
Now let’s subtract from :
In :
Now we plug in everything in :
Finally: Our number xyzuw is:
Using the Matrix inverse:
We get the augmented matrix
Making
Making
Finding:
We use row 2 since we have only two values in that row: is in row 2 and column 3, same for the 1 (row 2 column 5).
The determinant is , so we can calculate the inverse:
The tanspose of matrix is
The comatrix has 25 elements, we calculate 2 elements and let the students find the rest.
For of the comatrix:
For of the comatrix:
After all calculations we have:
The matrix is:
Now the solution is :
Cramer’s rule
We previously setup the solution and simplified
The student can calculate the following matrices:
The student can practice the matrix calculation for:
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