# How do you distinguish between an identity and an equation?

1. Jan 24, 2013

### tahayassen

If you're just given x2+y2=1, how would you know if it's an equation or an identity? Functions are identities, right?

2. Jan 24, 2013

It is an equation as soon as there is an "=" Functions are abstract objects that take an input and produce an output. What you have written is not a function, because it would have a multivalued output. One uses the word identity, in two cases. First case: With f(x)=0 can mean either for some x or for all x. In the second case one says f(x) is identically zero and you can use three bars as some kind of super equal sign. Second case you have a complicated term and you want to replace it with another complicated term like $\sin^2 x = 1 - \cos^2 x$ this type of equations as replacement rules are sometimes called identities.

3. Jan 24, 2013

### Studiot

If A is identical to B the we may replace A by B in all cases and all situations, without restriction.

If A equals B then we may only substitute B for A, subject to restrictions.

For example Distance = Speed x Time is an equation, but not an identity since it is subject to the condition that the speed is constant.

Last edited: Jan 24, 2013
4. Jan 24, 2013

### Staff: Mentor

There is usually some context around the equation if it's an identity. Sometimes people write this symbol, $\equiv$, when they write an identity.

For example, (x - 2)2 $\equiv$ x2 - 4x + 4.

The equation above is true for any real value of x.

An equation that is true only for specific values of the variable is called a conditional equation. Every equation that you're asked to solve is of this type.

For example, x2 - 2x = - 4.
This equation is true for only one value.

5. Jan 24, 2013

### WannabeNewton

A function is a type of relation so I don't know what you mean by it is an "identity". Perhaps you mean the identity map which is a special case of a function.

6. Jan 25, 2013

### disregardthat

Generally I'd say identities are statements, but equations are not.

When we mean sin^2x + cos^2x = 1 as an identity, we're saying something. Namely that this is true for all angles x.

But if I were to put 3x^2 + x = 2, i'm not saying anything. It can be an exercise, used in as a part of a mathematical statement or even a definition, but by itself it says nothing.

I could say "there is an x such that 3x^2+x=2", but I could also say "solve for x when 3x^2+x=2" or "let x be such that 3x^2+x=2". For identities, you'd always begin your sentence with "for all x: sin^2x+cos^2x = 1", quantifying each variable.

7. Jan 25, 2013

### Studiot

I think you have to be be very careful using just equations to show the difference between equality and identity. Several of the equations offered are suspect.

For example is the following equation an equality or an identity?

$$\frac{1}{2} + \frac{1}{2} = 1$$

If I cut a sheep in half and gave you both halves would that be identical to a whole sheep?

If I cut two sheep in half and gave you the back half of each one would that be the same as a whole sheep or the same as the first situation?

Another exmple is the equation

The sum of angles of a triangle = 180.

This equation is true for all plane triangles but does not make all triangles identical. It is not an identity.

Identity is also important without an explicit equation using numbers.

For example every equilateral triangle is similar but not identical. Similarity amongst triangles is not an identity.
However if the triangles are not only equilateral but have one side equal then they are congruent. This means they are the same whatever their orientation. Congruence is an identity.

Last edited: Jan 25, 2013
8. Jan 25, 2013

### Staff: Mentor

This statement is an identity.
This has nothing to do with the equation 1/2 + 1/2 = 1, which is purely a relationship with numbers. If you add additional context, such as that 1 represents 1 sheep, you are moving away from the mathematical meaning.
It's not saying that all (plane) triangles are identical - only that their angles add up to the same value. In the sense that this equation applies to all plane triangles, it is an identity - a statement that is always true.

9. Jan 25, 2013

### Staff: Mentor

This is incorrect. Conditional equations and identities are different kinds of statements. Inequalities are also statements.

In mathematics, a statement has a logical (i.e., true or false) value. A statement can be always true, true only under certain conditions, or never true (i.e., always false).
You are saying something; namely, that for some number(s), 3 times the square of the number plus the number is the same as 2. I can factor this equation and find the values of x that make it a true statement.

This equation is an example of a conditional equation, a kind of statement.

10. Jan 25, 2013

### Studiot

Good evening Mark. Your replies differ from the definitions given in my Collins Reference Dictionary of Mathematics. It has quite a bit to say about identities, the identity element, the identity function etc, offering 7 different mathematical cases of the word identity itself in all.

My congruent triangles corresponds to definition 5a.

It is true that a half plus a half can be an identity, and also corresponds to case5a but I gave conditions, I think you are invoking case1 incorrectly, "The property of being (another word for) the same individual". If value is all you are interested in then yes, but identity can be stronger than this.

Last edited: Jan 25, 2013
11. Jan 25, 2013

### Staff: Mentor

Hi Studiot,
I have a mathematics dictionary, but not at hand, so I can't compare. I agree that the word "identity" has different meanings in different contexts (e.g., identity function, identity element, etc.), but the context of the OP was in regard to equations. What I said about identities was specific to mathematical statements in the form of equations or inequalities.

12. Jan 25, 2013

### Studiot

I understand Tayahassen to be asking the very reasonable question what is the difference between an equality (equation) and an identity that we need to distinguish two different properties. And why (and when) do we use = and $\equiv$.

I have always understood that an identity is a stronger statement in some way and reflects the idea I put forward in post#3 that you can replace one side of an identity with the other without visible effect.

It is interesting that you write

and then complain when I provide context!

If you don't like counting sheep how about replacing them with a further geometric example?

Cut a triangle in 'halves'.

Call them half-A and half-B.

Let the cut divide the triangle so that half-A has two sides and half-B three.

Do this to a second triangle and add the Halves-A together or in my parlance substitute half A for half B in the original triangle.

You will now have a quadrilateral!

13. Jan 25, 2013

### disregardthat

As I explained, that would be if you put the "there exists an x" quantifier in front of the equation. I wouldn't call that part of the equation. Rather, the quantifier is added to it. The equation alone is not a complete mathematical statement, but has uses nevertheless. If you put an "for all x" in front, I'd call it an identity.

To be concise, in the statement (there exists an x: 2x=1), "there exists an x" is the quantifier, and "2x=1" is the equation. That doesn't make "2x=1" a statement.

What kind of conditions are you talking about here? Statements are either true or false, there are no conditions.

Last edited: Jan 25, 2013
14. Jan 25, 2013

### Staff: Mentor

My complaint was not that you provided context, but that what you provided was not related to what was asked in the OP, which was about equations.

And I'm not convinced that cutting a sheep in half says anything useful about the equation 1/2 + 1/2 = 1.

15. Jan 25, 2013

### Staff: Mentor

This is a good point, but might be a smidgen on the pedantic side. I would venture to say that most mathematics instructors would treat the qualifier "there exists an x such that..." as being implied. Logicians might quibble at this, but I doubt that mathematics instructors would.
I'll go out on a limb and say that most mathematics instructors would call this a conditional equation, which is a kind of statement.
Clearly we disagree here. I consider the equation 2x = 1 to be a statement. It is true if the variable happens to be 1/2, and is false otherwise.

16. Jan 25, 2013

### disregardthat

Clearly no disagreement of substance here.

But in my view you're talking about "For all x, if x = 1/2, then 2x=1", which is a (true) statement using the equation "2x=1". I don't really know of conditional statements, for me, statements are either true or false. "Conditional statements" seems like no more than a label for uncompleted statements. For the sake of clarity in this context uncompleted statements should not be mixed with proper ones.

17. Jan 25, 2013

### Staff: Mentor

FWIW, here's some of the wiki article on equations (http://en.wikipedia.org/wiki/Equation).
(I added the underline in two passages, below)
The example of the equation that is not an identity is what I'm calling a conditional statement.

18. Jan 25, 2013

### disregardthat

That's an informal article citing informal sources.

But to address their explanation, an equation is only a statement when there are no variables involved (in which case any quantifier is irrelevant). So asserting that x+3 is equal to 5 makes sense only when x is a predefined value, not when it's a variable. It is essential that all variables in an expression are bounded in order for it to be a statement (with a truth value).

See http://en.wikipedia.org/wiki/Sentence_(mathematical_logic)

In this context equations such as "2x +3 = 1" for a variable x are atomic sentences.

So "1=1", or "1+1 = 2" are statements, but "x=1" is not.

If, on the other hand, I say "let x = 3", then suddenly "x=3" is a (true) statement, but that's entirely different.

Last edited by a moderator: Jan 25, 2013
19. Jan 25, 2013

### Studiot

Hello, disregardthat, you have brought in the issue of 'completedness' in terms of qualifiers.

Equally I might observe that the statement $\forall$x is incomplete.

What x?

$\forall$x $\in$ R?

$\forall$x :→x>0?

etc

So where do you draw the line (stop)?

20. Jan 25, 2013

### Staff: Mentor

disregardthat,

In post #15, I said