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SUBMATH: MATRICES

This unit is about matrices and their application
SUBMATH: MATRICES 1

Matrices

In mathematics, a matrix (plural matrices) is a rectangular array of numbers, symbols, or expressions, arranged in rows and columns. Matrices are commonly written in box brackets.

Adding

To add two matrices: add the numbers in the matching positions:

Matrices: Matrix Addition

These are the calculations:
3+4=7 8+0=8
4+1=5 6−9=−3

The two matrices must be the same size, i.e. the rows must match in size, and the columns must match in size.

Example: a matrix with 3 rows and 5 columns can be added to another matrix of 3 rows and 5 columns.

But it could not be added to a matrix with 3 rows and 4 columns (the columns don’t match in size)

Negative

The negative of a matrix is also simple:

Matrix Negative

These are the calculations:

Subtracting

To subtract two matrices: subtract the numbers in the matching positions:

Matrix Subtraction

These are the calculations:
3−4=−1 8−0=8
4−1=3 6−(−9)=15

Note: subtracting is actually defined as the addition of a negative matrix: A + (−B)

Inverse of   a 2×2 matrix

What is the Inverse of a Matrix?

This is the reciprocal of a number:

Reciprocal of 8 is 1/8 and back again
Reciprocal of a Number

The Inverse of a Matrix is the same idea but we write it A-1

Reciprocal of A is A-inverse and back again

Why not 1/A ?  Because we don’t divide by a matrix! And anyway 1/8 can also be written 8-1

And there are other similarities:

When we multiply a number by its reciprocal we get 1

8 × (1/8) = 1

When we multiply a matrix by its inverse we get the Identity Matrix (which is like “1” for matrices):

A × A-1 = I

Same thing when the inverse comes first:

(1/8) × 8 = 1
A-1 × A = I

Identity Matrix

We just mentioned the “Identity Matrix”. It is the matrix equivalent of the number “1”:

Identity Matrix
A 3×3 Identity Matrix

  • It is “square” (has same number of rows as columns),
  • It has 1s on the diagonal and 0s everywhere else.
  • Its symbol is the capital letter I.

The Identity Matrix can be 2×2 in size, or 3×3, 4×4, etc …

Definition

Here is the definition:

The inverse of A is A-1 only when:

A × A-1 = A-1 × A = I

Sometimes there is no inverse at all.

2×2 Matrix

OK, how do we calculate the inverse?

Well, for a 2×2 matrix the inverse is:

matrix inverse 2x2 determinant

In other words: swap the positions of a and d, put negatives in front of b and c, and divide everything by the determinant (ad-bc).

Let us try an example:

matrix inverse 2x2 ex1

How do we know this is the right answer?

Remember it must be true that: A × A-1 = I

So, let us check to see what happens when we multiply the matrix by its inverse:

matrix inverse 2x2 ex2

And, hey!, we end up with the Identity Matrix! So it must be right.

It should also be true that: A-1 × A = I

Why don’t you have a go at multiplying these? See if you also get the Identity Matrix:

matrix inverse 2x2 ex3

Why Do We Need an Inverse?

Because with matrices we don’t divide! Seriously, there is no concept of dividing by a matrix.

But we can multiply by an inverse, which achieves the same thing.

Imagine we can’t divide by numbers …

… and someone asks “How do I share 10 apples with 2 people?”

But we can take the reciprocal of 2 (which is 0.5), so we answer:

10 × 0.5 = 5

They get 5 apples each.

The same thing can be done with matrices:

Say we want to find matrix X, and we know matrix A and B:

XA = B

It would be nice to divide both sides by A (to get X=B/A), but remember we can’t divide.

But what if we multiply both sides by A-1 ?

XAA-1 = BA-1

And we know that AA-1 = I, so:

XI = BA-1

We can remove I (for the same reason we can remove “1” from 1x = ab for numbers):

X = BA-1

And we have our answer (assuming we can calculate A-1)

In that example we were very careful to get the multiplications correct, because with matrices the order of multiplication matters. AB is almost never equal to BA.

A Real Life Example: Bus and Train

A group took a trip on a bus, at $3 per child and $3.20 per adult for a total of $118.40.

They took the train back at $3.50 per child and $3.60 per adult for a total of $135.20.

How many children, and how many adults?

First, let us set up the matrices (be careful to get the rows and columns correct!):

matrix inverse 2x2 bus

This is just like the example above:

XA = B

So to solve it we need the inverse of “A”:

matrix inverse 2x2 bus

Now we have the inverse we can solve using:

X = BA-1

matrix inverse 2x2 bus

There were 16 children and 22 adults!

The answer almost appears like magic. But it is based on good mathematics.

Calculations like that (but using much larger matrices) help Engineers design buildings, are used in video games and computer animations to make things look 3-dimensional, and many other places.

It is also a way to solve Systems of Linear Equations.

The calculations are done by computer, but the people must understand the formulas.

Order is Important

Say that we are trying to find “X” in this case:

AX = B

This is different to the example above! X is now after A.

With matrices the order of multiplication usually changes the answer. Do not assume that AB = BA, it is almost never true.

So how do we solve this one? Using the same method, but put A-1 in front:

A-1AX = A-1B

And we know that A-1A= I, so:

IX = A-1B

We can remove I:

X = A-1B

And we have our answer (assuming we can calculate A-1)

Why don’t we try our bus and train example, but with the data set up that way around.

It can be done that way, but we must be careful how we set it up.

This is what it looks like as AX = B:

matrix inverse 2x2 bus

It looks so neat! I think I prefer it like this.

Also note how the rows and columns are swapped over
(“Transposed”) compared to the previous example.

To solve it we need the inverse of “A”:

matrix inverse 2x2 bus
It is like the inverse we got before, but
Transposed (rows and columns swapped over).

Now we can solve using:

X = A-1B

matrix inverse 2x2 bus

Same answer: 16 children and 22 adults.

So matrices are powerful things, but they do need to be set up correctly!

Multiply by a Constant

We can multiply a matrix by a constant (the value 2 in this case):

Matrix Multiply Constant

These are the calculations:
2×4=8 2×0=0
2×1=2 2×−9=−18

We call the constant a scalar, so officially this is called “scalar multiplication”.

Multiplying by Another Matrix

To multiply two matrices together is a bit more difficult … read Multiplying Matrices to learn how.

VIDEO TUTORIAL

Solving Systems of Linear Equations Using Matrices

Assignment

SUBMATH: Matrices Assignment

ASSIGNMENT : SUBMATH: Matrices Assignment MARKS : 10  DURATION : 1 week, 3 days

 

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