We compose A-1 instead of 1A due to the fact that we don"t divide by a matrix!

And over there are various other similarities:

When we multiply a matrix by its inverse we gain the identification Matrix (which is favor "1" because that matrices):

Identity Matrix

We simply mentioned the "Identity Matrix". It is the matrix indistinguishable of the number "1":

It is "square" (has same variety of rows as columns),It has actually 1s on the diagonal and 0s all over else.Its prize is the funding letter I.

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The identification Matrix have the right to be 2×2 in size, or 3×3, 4×4, and so on ...


Here is the definition:

(Note: composing AA-1 method A time A-1)

2x2 Matrix

OK, how do us calculate the inverse?

Well, for a 2x2 procession the inverse is:

In various other words: swap the positions of a and also d, placed negatives in front of b and also c, and also divide everything by ad−bc .

Note: ad−bc is called the determinant.

Let us shot an example:

How perform we recognize this is the best answer?

Remember it must be true that: AA-1 = I

So, let us examine to see what happens when we main point the matrix by its inverse:

And, hey!, we finish up with the identification Matrix! for this reason it have to be right.

It need to also it is in true that: A-1A = I

Why don"t you have actually a walk at multiply these? see if you also get the identification Matrix:

Why perform We need an Inverse?

Because v matrices we don"t divide! Seriously, over there is no ide of separating by a matrix.

But we deserve to multiply by one inverse, which achieves the same thing.

Imagine we can"t divide by number ...

... And also someone asks "How perform I re-publishing 10 apples v 2 people?"

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

10 × 0.5 = 5

They gain 5 apples each.

Say we want to discover matrix X, and also we recognize matrix A and also B:

XA = B

It would be quite to division both political parties by A (to acquire X=B/A), yet remember we can"t divide.

But what if we multiply both political parties by A-1 ?

XAA-1 = BA-1

And we understand that AA-1 = I, so:

XI = BA-1

We deserve to remove i (for the same reason we deserve to remove "1" from 1x = abdominal for numbers):

X = BA-1

And we have actually our price (assuming we deserve to calculate A-1)

In that instance we were very careful to acquire the multiplications correct, because with matrices the stimulate of multiplication matters. Abdominal is practically never equal to BA.

A genuine Life Example: Bus and Train


A group took a expedition on a bus, at $3 per child and $3.20 every adult because that a total of $118.40.

They take it the train ago at $3.50 every child and also $3.60 every adult because that a total of $135.20.

How many children, and also how many adults?

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


This is as with the example above:

XA = B

So to deal with it we need the train station of "A":

There to be 16 children and 22 adults!

The answer almost appears favor magic. Yet it is based on great mathematics.

Calculations favor that (but utilizing much larger matrices) help Engineers design buildings, are used in video clip games and computer animations come make points look 3-dimensional, and also many various other places.

It is additionally a way to settle Systems of linear Equations.

The calculations space done by computer, however the civilization must know the formulas.

Say the we space trying to uncover "X" in this case:

AX = B

This is various to the example above! X is currently after A.

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

So exactly how do we resolve this one? utilizing the very same method, yet 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 deserve to calculate A-1)

Why don"t we shot our bus and also train example, however with the data set up that method around.

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

This is what it looks favor as AX = B:

It looks for this reason neat! ns think I like it choose this.

Also note just how the rows and also columns room swapped over("Transposed") compared to the vault example.

To settle it we require the train station of "A":

It is favor the train station we gained before, butTransposed (rows and also columns swapped over).

Now we can solve using:

X = A-1B

Same answer: 16 children and 22 adults.

So matrices are powerful things, however they do must be collection up correctly!

The Inverse may Not Exist

First of all, to have actually an inverse the matrix should be "square" (same variety of rows and also columns).

But additionally the determinant can not be zero (or we end up separating by zero). How around this:

24−24? That equates to 0, and 1/0 is undefined.We cannot go any further! This matrix has no Inverse.

Such a matrix is referred to as "Singular",which only happens once the determinant is zero.

And it renders sense ... Look in ~ the numbers: the 2nd row is just dual the first row, and also does not include any brand-new information.

And the determinant 24−24 lets us understand this fact.

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(Imagine in our bus and train instance that the price on the train to be all exactly 50% higher than the bus: so now we can"t figure out any kind of differences between adults and also children. There requirements to it is in something to collection them apart.)

Bigger Matrices

The inverse of a 2x2 is easy ... Compared to bigger matrices (such together a 3x3, 4x4, etc).

For those larger matrices there space three main methods to occupational out the inverse:


The inverse of A is A-1 only once AA-1 = A-1A = ITo uncover the station of a 2x2 matrix: swap the positions of a and d, put negatives in former of b and also c, and also divide every little thing by the determinant (ad-bc).Sometimes there is no inverse in ~ all
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