Finding the Domain of a Function
Date: 09/16/97 at 23:00:28 From: Shelly Breeding Subject: Finding the domain of a function I had this problem on a quiz and I have no idea how to figure out the domain of this function: f(x) = (4+x)/(x^2-9) Can you please show me how to figure out the domain, maybe by graphing it?
Date: 11/10/97 at 11:45:53 From: Doctor Sonya Subject: Re: Finding the domain of a function Hi Shelly, The domain of a function is the set of numbers that you can plug into the function and get out something that makes sense. This is also called the set of numbers for which this function is defined. In this case it is the set of all x, such that f(x) is a number. One way to try to find the domain is to try a couple of numbers just to get an idea of what might work. (1) If we plug in 4, we get f(4) = (4+4)/(4^2 - 9) = 8/7. (2) If we plug in 0, we get f(0) = (4+0)/(0^2 - 9) = 4/-9 = -(4/9) (3) If we plug in 3, we get f(3) = (4+3)/(9-9) = 7/0 = Oops! I'm sure you've been told in class that you can't divide by 0, so 7/0 is an answer that doesn't make sense, and one would say that f(x) is not defined at x = 3. You would find that the same holds for x = -3. Okay, so far we think that the domain of f(x) is all numbers except 3 and -3. But how do we know this is correst? In principle you would have to try every real number other than these two to be sure. In practice it is much easier. You just need to ask yourself a question: Which is the only way this function can be undefined? You know that you always get a fraction out of this function. (If you get a whole number n, you can think of it as the fraction n/1.) The only fractions that are undefined are those with 0 in their denominators. We have a zero in the denominator only when: x^2 - 9 = 0 or only when x = 3 or x = -3 This means that the function is undefined only at these two numbers, so its domain is all real numbers but 3 and -3. I hope that this has helped. -Doctors Marko and Sonya, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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