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Homework Help: A quick stats question

  1. Nov 7, 2007 #1
    1. The problem statement, all variables and given/known data
    If X is a discrete random variable with p.d.f. f(x) = {cx if x=1,2,3,4 ; 0 otherwise} where c is a constant.
    Find P(1< X =< 3.5)

    2. Relevant equations
    I solved for c:
    f(x) = 1 = c(1+2+3+4)
    10c = 1
    c = 1/10

    3. The attempt at a solution
    I have the following solution:
    P(1< X =< 3.5) = f(3.5) - f(1)
    = c(3.5) - c(1)
    = c(2.5)

    I am not sure if this is correct. Please help!

  2. jcsd
  3. Nov 7, 2007 #2
    f(x) = 1 = c(1+2+3+4) why ?
    isnt it integral of f(x)=1?
    i could be totaly off though!
  4. Nov 7, 2007 #3
    hmm, possibly... we're not supposed to use calc in my course, or would there be another way of solving it?
  5. Nov 7, 2007 #4
    no idea, im in a class too, and apparently i have to do that to somethign with 2 variables, and im so confused! :P sorry i can't help more
  6. Nov 7, 2007 #5
    aw, no problems... this question is so frustrating because it's really difficult to find examples online, and I don't think we did a single example in class of this. I don't want to just guess the answer, but I may have to!
  7. Nov 7, 2007 #6


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    Science Advisor
    Homework Helper

    X is discrete. It can only take on integer values 1, 2, 3, 4. You are violating this condition in your calculation.

    Does this help?
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