1. Not finding help here? Sign up for a free 30min 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!

Question 19 - quadratic probility problem

  1. Apr 4, 2007 #1
    [​IMG]

    a) (i) (x-14)(2x-7)
    (ii) x = 14 or x = 3.5

    b)
    i) [tex]\frac{7}{n+7}[/tex]
    ii) Take n to be 8

    [tex]\frac{7}{8+7}[/tex]
    [tex]\frac{7}{15}[/tex] that DOESN'T round down to [tex]\frac{2}{5}[/tex]

    Is that all correct so far?
    If so I will post the next (really hard) question)...

    Thanks
     
  2. jcsd
  3. Apr 4, 2007 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Why take n to be 8? Nothing is said about any value for n.
    If 7/(n+7)= 2/5, SOLVE for n. What happens?
     
  4. Apr 4, 2007 #3
    [tex]\frac{7}{n+7}[/tex] = [tex]\frac{2}{5}[/tex]

    cross multiply!

    [tex]\frac{35}{2n+14}[/tex]

    now where?
     
  5. Apr 4, 2007 #4
    Um...you're leaving out the equality part of the equation. Solve for n.
     
  6. Apr 4, 2007 #5
    Alternatively, you can note that you have 35/(2n + 14) = 1. 35 is odd. 2n + 14 is even. Strange, isn't it?
     
  7. Apr 4, 2007 #6
    uh? How does that equal 1?
     
  8. Apr 4, 2007 #7
    Look, you're essentially supposed to say, "Suppose Bill is right. Suppose 7/(n + 7) = 2/5. Then such and such would follow." Why would the conclusion be a problem?
     
  9. Apr 4, 2007 #8
    [tex]\frac{35}{2n+14}[/tex] = 1

    now I need to get N on it's own (don't know how- plz show). But im guessing that n is greater than 3/5 so it CANT be right?
     
  10. Apr 4, 2007 #9

    danago

    User Avatar
    Gold Member

    Multiply both sides of the equation by 2n + 14, so you end up with:

    35 = 2n + 14
     
  11. Apr 4, 2007 #10
    silly me....

    2n = 35-14
    n = 10.5
    EDIT: Which as a fraction is 10/1/2 which DOESN'T equal 2/5...am i right yet. I doubt thats right...
     
  12. Apr 4, 2007 #11
    You're getting there. What's the problem with n being 10.5. Look at your original assumptions. What are you tacitly assuming about the original n balls?
     
  13. Apr 5, 2007 #12
    you cant have 1/2 a ball... :bugeye:

    am I right or am I right
     
    Last edited: Apr 5, 2007
  14. Apr 6, 2007 #13
    Okay here is the rest of the question

    [​IMG]
     
  15. Apr 6, 2007 #14

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Okay, what are your ideas on part (c)?
     
  16. Apr 6, 2007 #15
    (probability of taking white x probability of taking yellow) + (probability of taking yellow x probality of taking white)

    Is that somthing to go from?

    Thanks
     
  17. Apr 6, 2007 #16

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, I'd go with that.
     
  18. Apr 6, 2007 #17
    I got it down now to [tex]\frac{14n}{2n^{2}+28n+98}[/tex] = [tex]\frac{4}{9}[/tex]

    is that right so far?

    EDIT: is the question is says -28n but ive got +28n


    EDIT 2: O no it must be this so far

    [tex]\frac{14n}{n^{2}+14n+49}[/tex] = [tex]\frac{4}{9}[/tex]
     
  19. Apr 6, 2007 #18
    got it!

    [tex]\frac{4n^{2} + 56n + 196}{2}[/tex] = [tex]\frac{14n * 9}{2}[/tex]
    [tex]\2n^{2} + 28n + 98[/tex] = [tex]68n[/tex]


    [tex]\2n^{2} - 35n + 98 = 0[/tex]
     
  20. Apr 6, 2007 #19
    and the answer to d must be 1/9
    x must be 14 as you cant have 3.5 balls
     
  21. Apr 6, 2007 #20

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    All sounds good to me, well done :approve:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Question 19 - quadratic probility problem
  1. Quadratic problem (Replies: 4)

  2. A quadratic problem (Replies: 3)

  3. Quadratic Problem (Replies: 7)

  4. Quadratic Question (Replies: 7)

  5. Quadratic problem (Replies: 4)

Loading...