if A=B , is it the same to say B=A? if it's, then why? what's confusing me A=B isn't in writings the same B=A so how we determined that A=B is the same as B=A!!!!!

I know it's equal, but **who said we just care on equal?** maybe also on the **order of writing the elements** of two sides of equation .. who said not?!

Since another question hints that you know something about computer programming, it may be worth pointing out that

**"=" can be used in different ways**. Sometimes order doesn't matter, sometimes it does.

"Who says?" This is just an implicit agreement among users of the notation, in a particular context -- like all language! If you choose to be part of a community, and communicate with them, you use the words and symbols they use, in the way they use them.

In math, "=" by itself always means merely "is equal to". When we say that A = B, it means that quantities A and B are equal -- both quantities play the same role; it doesn't matter which order you put them. You can think of it as bidirectional, or symmetrical. (It can also be used in a definition, as in "let n = 3", which is not symmetrical; but even there, "=" just means these two quantities are equal; it is the sentence in which it is found that changes it from a mere statement of equality to a definition.)

This is what all mathematicians accept as the meaning. And this is what the symbol has meant since it was first invented, as described

here.

But some programming languages use "=" in a different way, meaning "is assigned", similar to the usage in definitions. There, writing A = B means "put the current value of variable B into variable A". This is one-directional; it actually tells the computer to do something to variable A, and not to B. For example, in programming you can say "x = x + 1", which in math would be nonsense, but in a program changes the value of x by adding 1 to it.

Similarly, in English, the word "is" can be used in different ways. Taking pka's example, "Samuel Clemens is Mark Twain" means that they are two names for the same person; but "Samuel Clemens is an author" describes one aspect of Clemens, and identifies only one of many people who are authors -- it doesn't equate

*all that Samuel Clemens is* with

*all authors*. Or, returning to math, we can say "a square is a rectangle" but "a rectangle is a square" means something very different; this usage is asymmetrical.

But many students think of "=" in math as if it meant "has the answer", as in "2 + 3 = 5" meaning "if you add 2 + 3, the answer you get is 5". That is a misunderstanding; it really means merely that 2 + 3 and 5 are two "names" for the same quantity. When you move from arithmetic into algebra, you have to leave that earlier way of thinking behind.