1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Simple Definition of a "Math Restriction" (Layman's Terms)?

  1. Sep 18, 2014 #1
    1. The problem statement, all variables and given/known data

    For which value(s) of x is each expression not defined?

    2. Relevant equations

    x^2+3x-6 divided by x^2-x-12

    3. The attempt at a solution

    The answer that was given turned out to be:

    x^2+3x-6 divided by (-4)(x+3)

    Restrictions: x ≠ 4,-3


    I'm trying to learn what a restriction is and what the point of them are, but I've only come across complicated answers from all over the web. Some of the questions I've encountered apparently don't have any restrictions at all as well.

    If I can learn what these are then Rational Expressions will become that much easier for me, thank you.
  2. jcsd
  3. Sep 18, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The common things that restrict the domain for real functions are:
    1. Denominator can't be zero (as in this example)
    2. Argument under a square root sign can't be negative
    3. Argument of logarithm must be non-negative.
  4. Sep 19, 2014 #3


    User Avatar
    Science Advisor

    No, it's not. The denominator is (x- 4)(x+ 3).

    Right- if x= 4 then x- 4= 0. If x= -3 then x+ 3= 0. In either case, (x- 4)(x+ 3) would be 0 and you cannot divide by 0.
    For rational expressions, they are all based on the fact that division by 0 is not defined, a simple fact of arithmetic.

    For other kinds of functions there may be other restrictions. For example, as long as you are working with the real number system, you cannot take the square root of a negative number. And a logarithm can only be applied to positive numbers.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Threads - Simple Definition Math Date
Simple Interest: Number of Years Oct 21, 2017
Simple Interest Oct 18, 2017
Simple Interest help -- investment in his savings account matured to $4,106.32 Oct 18, 2017
Simple Interest % help Oct 18, 2017