Question 453994
Rule:

1. Look at all the first coordinates

2. Then ask the question: Are all those different?

3. If the answer is yes, then this is a function but if 
the answer is no, then it is not a function.

Let's try it out on

1-{(5,7),(-2,1),(5,0),(-3,6)}

1. Look at all the first coordinates 5, -2, 5, -3

2. Then ask the question: Are all those different?

3. The answer is no because there are two 5's, 

So it is NOT a function.

------------------------------------

Now let's try it out on

2-{(-2,-1),(4,5),(4,-2),(3,4)}

1. Look at all the first coordinates -2, 4, 4, 3

2. Then ask the question: Are all those different?

3. The answer is no because there are two 4's, 

So it is NOT a function.

------------------------------------
Now let's try it out on

3-{(0,4),(3,6),(-1,2),(4,0)}

1. Look at all the first coordinates 0, 3, -1, 4

2. Then ask the question: Are all those different?

3. The answer is yes because none of them are the same. 

So it IS a function.

-----------------------------------

Easy, huh?  Just look and see if all the first coordinates
are all different or whether some are the same. If they're
all different then you have a function. If some are the
same you don't.  [By the way, it doesn't matter at all about 
the second coordinates.  They can be the same or different. 
We only need to look at the first coordinates.]

Edwin</pre>