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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,664

Is it A-B or is it A+(-B) ?

(I am making some pages on Matrices.)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi Mathsisfun;

Yes, matrix subtraction is a legal operation. It is achieved by element by element subtraction. As long as A and B are the same size.

A + ( - B ) is also legal, the addition is possible provided A and B are the same size.

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.**** Always satisfy the Prime Directive of getting the right answer above all else.**

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**ZHero****Real Member**- Registered: 2008-06-08
- Posts: 1,889

Hi MathIsFun!

I think that you may find the following link to be useful...

play through the replay buttons

If two or more thoughts intersect, there has to be a point!

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,664

Thanks!

I do seem to recall that subtraction is not defined as such for matrices, it is in fact addition of a negative. And that website agrees, but then goes on to do subtraction as an operation ... so that leaves it nicely **un**resolved.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**bobbym****bumpkin**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 109,606

Hi MathsisFun;

That is true, I saw that page. I am not much on definitions, to me adding a negative is subtraction. But for the purposes of mathematical correctness you could include their definition.

**In mathematics, you don't understand things. You just get used to them.****If it ain't broke, fix it until it is.**** Always satisfy the Prime Directive of getting the right answer above all else.**

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**cool_jessica****Member**- Registered: 2012-06-26
- Posts: 15

Hello,

Matrix subtraction is a legal operation but the elements needs to be same in both the matrix. for eg a @*2 matrix can be substracted only from 2*2 matrix and not 3*3 matrix.

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 16,037

MathsIsFun wrote:

Thanks!

I do seem to recall that subtraction is not defined as such for matrices, it is in fact addition of a negative. And that website agrees, but then goes on to do subtraction as an operation ... so that leaves it nicely

unresolved.

Adding a negative and subtraction are different operation that achieve the same thing, so I guess either is okay.

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**noelevans****Member**- Registered: 2012-07-20
- Posts: 236

Hi MathIsFun!

In the field axioms for the real numbers subtraction and division are not mentioned, only addition and multiplication.

Subtraction and Division are INTRODUCED via definitions: x-y is x plus the opposite of y and x/y is x times the multiplicative inverse of y. x-y = x+(-y) where we use "-y" for the opposite of y. So subtraction is a "secondary" operations, not really necessary, but quite handy at times.

But for reciprocals we have no such concise notation. 1/y suggests fractions and uses the symbol

"/" that is often interpreted also as division. And x^(-1) is defined typically as 1/y. If we had a

SIMPLE notation for RECIPROCALS (such as /x ) then we could define division in a way that would obviously be analogous to the definition of subtraction.

x - y = x + (-y) vs x/y = x*(/y)

We perhaps get the "-x" from "shortening down 0-x" so that by analogy we could get

"/x" from "shortening down 1/x".

But back to linear algebra:

Most linear algebra books follow the same pattern for two matrices of the SAME dimensions: A-B is defined as A plus the opposite of B where the opposite of B is the same as B except every entry in -B is the opposite of the corresponding entry in B. So by definition A-B=A+(-B). Some of the books probably just assume the reader understands the "stepping up" of the definition via the field axioms to the situation with matrices.

A similar situation holds for "division" of matrices. If we have the inverse of a matrix A (written A^(-1) ) then we can multiply A*A^(-1) to get an IDENTITY matrix, where the identity matrix

functions like the multiplicative identity 1 in the reals. But in the reals only zero has no reciprocal.

There are many matrices that have no multiplicative inverse, for example, those for which their

determinant is zero.

So the choice is up to you whether you want to write A-B vs A+(-B). If one does not allow

subtraction then they are stuck with A+(-B).

Have a stupendous day!

Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).

LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

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