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!

When find LCD for a rational expressions.

  1. Feb 13, 2005 #1
    Ok I have to find the Lowest Common Denominator for 3 ration expressions.
    I dont think the numerators are important so Ill just leave them out. The denominators are

    x^2 - 4 and x -2 I got the LCD as x-2. correct?

    When finding the LCD in expressions like this you just have to factor and pick the term that they both have in common right?
     
  2. jcsd
  3. Feb 13, 2005 #2

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Nope,u have to pick just that:the lowest common denominator.Think of the numbers:what is the lowest common multiple of 6 and 3...?Is it 3??

    Daniel.
     
  4. Feb 13, 2005 #3
    Well the first one comes out to (x-2) and (x+2) while the 2nd one just stays as (x-2). So isnt the only CD they have (x-2)?
     
  5. Feb 13, 2005 #4

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    No,because the common denominator has to be a common multiple of the "old" denominators...That's why i gave the example with the numbers...Instead of #,you have polynimials...

    Daniel.
     
  6. Feb 13, 2005 #5
    Would you mind just giving me the answer for the one I mentioned? I have 3 more problems just like it and right now Im seriously not getting it.
     
  7. Feb 13, 2005 #6

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    It would really matter for you to know/understand why the answers is the one that it is,namely [tex] x^{2}-4 [/tex].

    Daniel.
     
  8. Feb 13, 2005 #7
    Ok I think I get it now. You take x-2 from the first one and x-2 from the 2nd one and multiply them right? But wouldnt that end up x^2 -4x + 4? Why do I take the x+2 from the first one?

    EDIT: I take all the DIFFERENT factors right? So that they each get a part?
     
  9. Feb 13, 2005 #8

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Nope,it has to be the smallest common denominator,as i said the smalles common MULTIPLE OF THE 2 DENOMINATOS.The smallest one for [itex] x^{2}-4 [/itex] is [itex] x^{2}-4 [/itex] and that's that...

    That simple multiplication between the denominators would be valid if the 2 polynomials would be prime one wrt another,which is not the case in here...

    Daniel.
     
  10. Feb 13, 2005 #9
    So I can essentially just multiply the 2 denominators of any problem like this and still get the correct answer? Or factor out the 2 denominators and multiply all the factors with each other, crossing out all the factors that were in both?
     
  11. Feb 13, 2005 #10

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    Yes,you finally got it...That "crossing out" doesn't mean eliminating,just "counting" only once in the product,okay?
    In your case,you'd have to count "x-2" only once.

    Daniel.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook