Relations versus FunctionsDate: 10/27/98 at 22:58:00 From: NICOLE Subject: Functions Dear Dr. Math, In Algebra 2, the chapter we are working on now is about functions and linear functions. How can you tell whether an equation is a function or just a relation? Given the domain and the range, how do you decide whether or not a relation is a function? Please help. Date: 10/28/98 at 09:56:02 From: Doctor White with Doctor Teeple Subject: Re: Functions Nicole: First, all sets of ordered pairs are relations. A function is a relation with a certain criterion. The criterion is that for every domain value put into the function, the function will create only one range value. We think of domain elements as being values we put into a function to produce a result. For an example of a function, let's use a ruler. You may not think of a ruler as a function, but each time you measure an object, you assign the object a number. So in this case, the elements in your domain are the objects you measure. The range consists of the possible numbers you can get when you measure. In the ruler example, the range is the positive real numbers, since you can't have negative measurements. When you indicate your function, to be really clear it is best to define your domain and your range, in addition to your function. For example, suppose we measure the length of a pen, the length of a table, and the length of a rug. The domain is the set A = {pen, table, rug}. Then we would say something like: "Let f be a function from the set A to the reals, defined by measuring the object with a ruler." For a more mathy example, we could say: "Let f:R->R be defined by f(x) = 5x-2", where the notation f:R->R means: the function f from the domain R (reals) to the range R. One thing we have have to watch out for in functions is to be sure that an element in the domain doesn't go to two elements in the range. I'll give you an instance using our ruler example. Suppose we are trying to figure out the length of the pen. We take the ruler and measure 6.3. But then someone else measures and says that he thinks the answer is 16. What's going on here? Well, we figure out that we measured in inches, and the other person measured in centimeters. So who's right? They are both measurements, so by our "function" we are both right. That can get pretty confusing, and we need to make sure that each element maps to only one element in the range. We can show this a couple of ways. Numerically: Let x = y^2. If x = 1, then y can equal 1 and -1. One domain produced two ranges. This is NOT a function. Graphically: If you sketch the graph of the relation, you can use the vertical line function test. Sketch the graph. If you can draw a vertical (up and down) line through any x-value, and it hits the graph at more than one place then it is NOT a function. If it hits it only once then it IS a function. I hope all of this helps you see this a little better. Please note that this is only one of the conditions we have to check to make sure that a relation is a function. For more information, please see the following entry in our archives: Relations on a Set, as Mappings http://mathforum.org/dr.math/problems/turner.7.19.96.html Come back to see us soon. If you need more information, let us know. - Doctors Teeple and White, The Math Forum http://mathforum.org/dr.math/ |
Search the Dr. Math Library: |
[Privacy Policy] [Terms of Use]
Ask Dr. Math^{TM}
© 1994- The Math Forum at NCTM. All rights reserved.
http://mathforum.org/dr.math/