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: Probability given a cumlative funcion

  1. Feb 29, 2008 #1
    First determine the distribution function F(x)

    f(x)= x/16 for 0<x<4
    1/2-x/16 for 4[tex]\leq[/tex]x<8
    0 for elsewhere

    So I determined this & came up with the function (which is correct)

    F(x)= 0 for x<o
    x^2/32 for 0<x<4
    x/2-x^2/32-1 for 4[tex]\leq[/tex]x<8
    1 for x[tex]\geq[/tex]8

    Then there is the question P(X>10)

    Now, my thoughts is that from the original problem, the probability that x is greater than 8 is zero, & therefore, it should be zero... But, I got the question wrong & the professor stated that P(X>10) is 1. Which doesn't make a whole lot of sense to me, but she explained it that since it doesn't have an upper bound, it must be 1. Any ideas here? We had a homework question that stated a similar P(11<x<12) but here, it was zero because she stated it was bounded outside of the range.
     
  2. jcsd
  3. Feb 29, 2008 #2

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    I'm with you. P(X>10) is 0. P(X<10) is 1. I don't see how you could interpret the problem otherwise.
     
  4. Mar 2, 2008 #3
    Do you think given that it is a big X & not a little x has anything to do with it? I could see if it was F(X>10) perhaps... the notation always ends up getting the best of me.
     
  5. Mar 2, 2008 #4

    HallsofIvy

    User Avatar
    Science Advisor

    Could you state the problem exactly it is given? Yes, P(x> 10) is 0 but you titled this "cumulative" function and have F(x)= 1 for [itex]x\ge 8[/itex].
     
  6. Mar 2, 2008 #5
    It is stated "Determine the distribution function, F(x) of the random variable X whose probability density is..." what I outlined first.

    It then says "Use the information about F(x) in the previous question to determine" (which is the cumulative function I outlined)
    a) P(X<6) which I did & showed the work 6/2-36/32-1 = 7/8 (which was correct)
    b) P(X>10) which I showed the work 1-1=0 (which was marked wrong)
    in class she specifically said b) was suppose to be 1. 1-0=1
     
  7. Mar 2, 2008 #6

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    I think this is all a misunderstanding. If P(X<6) is 7/8 then P(X>10) CANNOT be 1. They add to a number larger than 1 and they are mutually exclusive. That would just plain be silly. Please ask your teacher to explain. I can't.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook