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!

Homework Help: Find derivative using Quotient rule

  1. Oct 5, 2008 #1
    1. The problem statement, all variables and given/known data

    f(x) = 6-5x-x2 / x2-1
    A) -5-5x / x2-1

    B) 5 / (x+1)2

    C) -5-5x / (x2-1)2

    D) -5 / (x+1)2

    E) -5+5x / x2-1

    F) None of the above

    3. The attempt at a solution
    (x2)(-5-2x)-(6-5x-x2)(2x) / (x2-1)2

    Simplified to:

    5x2-10x+5 / x4-2x2+1

    But it's not one of the answer choices. I tried simplifying it even further to:

    5(x2-2x) / x4-2x2+1
    5(x2-2x) / (x2-1)2

    But I can't see where else to go with it.
  2. jcsd
  3. Oct 5, 2008 #2
    Ok so you're taking a quiz or exam? So I think it's best to wait till you're done :)
  4. Oct 5, 2008 #3
    It actually is just homework
  5. Oct 5, 2008 #4
    I don't see much hw with multiple choice, so yeah ...
  6. Oct 5, 2008 #5
    I don't even get the benefit of the doubt? I can guarantee you that this is just a homework problem.

    But if you're not going to believe or help me, can you at least tell me if I'm headed in the right direction or did I miss something?
  7. Oct 5, 2008 #6
    "(x^2)(-5-2x)-(6-5x-x^2)(2x) / (x2-1)^2"

    the first (x^2) should be (x^2 - 1)

    it could also be possible that the answers you are given are in a simplified form. What are the answers?
  8. Oct 5, 2008 #7
    These are the choices:

    (A) -5-5x / x2-1

    (B) 5 / (x+1)2

    (C) -5-5x / (x2-1)2

    (D) -5 / (x+1)2

    (E) -5+5x / x2-1

    (F) None of the above


    Yea, I forgot to put the "-1", but I did do it on my paper.

    (x2-1)(-5-2x)-(6-5x-x2)(2x) / (x2-1)2

    I got:
    -5x2+5-2x3+2x-12x+10x2+2x3 / x4-2x2+1

    Simplified to:
    5x2-10x+5 / x4-2x2+1


    5(x2-2x+1) / (x2-1)2
    5(x-1)2 / (x2-1)2

    But I don't know where to go from there
  9. Oct 5, 2008 #8
    5(x-1)2 / (x2-1)2 is correct thusfar. Just notice that (x2-1) is a difference of two squares. Change it to (x+1)(x-1) and your answer simplifies to (B), which should be correct.
    Last edited: Oct 5, 2008
  10. Oct 5, 2008 #9
    I completely missed that. Thanks!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook