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Continuous random variable

  1. Oct 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Suppose the distance X between a point target and a shot aimed at the point in a coin-operated target game is a continuous random variable with pdf

    f(x) = { k(1−x^2), −1≤x≤1
    0, otherwise.

    (a) Find the value of k.

    (b) Find the cdf of X.

    (c) Compute P (−.5 < X ≤ .5).

    (d) Find the expected distance between a point target and a shot aimed.



    3. The attempt at a solution
    a) [itex] k\int_{-1}^1(1-x^2)dx [/itex]

    [itex]= k[\int_{-1}^1dx-\int_{-1}^1x^2dx] [/itex]

    [itex]= k[x\Big|_{-1}^1-\frac{1}{3}x^3\Big|_{-1}^1] [/itex]

    = k(2-2/3) = 1

    k(4/3) = 1

    k = 3/4


    b) [itex] \frac{3}{4} \int_{-1}^X(1-x^2)dx [/itex]


    c) [itex] \frac{3}{4}[x\Big|_{-.5}^{.5}-\frac{1}{3}x^3\Big|_{-.5}^{.5}] [/itex]

    = (3/4)(1-(1/3)[2(1/3)(1/8)])

    = (3/4)(1-1/36)
    = .7292

    d) [itex] \frac{3}{4}\int_{-1}^1x(1-x^2)dx [/itex]

    [itex] =\frac{3}{4}\int_{-1}^1(x-x^3)dx [/itex]

    [itex]= \frac{3}{4}[\int_{-1}^1xdx-\int_{-1}^1x^3dx] [/itex]

    [itex]= \frac{3}{4}[\frac{1}{2}x^2\Big|_{-1}^1-\frac{1}{4}x^4\Big|_{-1}^1] [/itex]

    = 0


    am I doing this right?
     
  2. jcsd
  3. Oct 19, 2015 #2

    SteamKing

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    For b), instead of expressing the cdf as an integral, you should actually carry out the integration and express the cdf as a function of X.
     
  4. Oct 19, 2015 #3

    Ray Vickson

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    Not for (d), no. You computed ##E X##, but what is wanted is ##E |X|##. Also, I get a different answer for (c).
     
  5. Oct 19, 2015 #4
    for part b)
    [itex] F(X) = \frac{3}{4}[x\Big|_{-1}^X-\frac{1}{3}x^3\Big|_{-1}^X] [/itex]

    [itex] F(X) = \frac{3}{4} [(X+1) - \frac{1}{3} (X^3 +1)] [/itex]

    [itex] F(X) = \frac{3}{4}[X + 1 - \frac{X^3}{3} - \frac{1}{3}] [/itex]

    [itex] F(X) = \frac{3}{4}[X - \frac{X^3}{3} + \frac{2}{3}] [/itex]

    [itex] F(X) = \frac{3X}{4} - \frac{X^3}{4} + \frac{1}{2} [/itex]

    part c) I made a arithmetic error. it comes down to (3/4)(11/12) = .6875

    part d)
    I am not quite sure how to do this. plugging in |x| whereever there is an x gives me the same answer. should I switch the limits of integration from 0 to 2 instead of -1 to 1?
     
  6. Oct 19, 2015 #5

    Ray Vickson

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    No. Without doing any calculations you can see why ##EX = 0##: it is because ##x f(x)## is an odd function on ##[-1,1]## so integrates to zero automatically. That is NOT the case for ##|x| f(x)##, because this not now an odd function on ##[-1,1]##. I won't say any more.
     
  7. Oct 19, 2015 #6
    d) [itex] \frac{3}{4}\int_{-1}^1|x|(1-x^2)dx [/itex]

    [itex] =\frac{3}{4}\int_{-1}^1(|x|-|x|x^2)dx [/itex]

    [itex]= \frac{3}{4}[\int_{-1}^1|x|dx-\int_{-1}^1|x|x^2dx] [/itex]

    [itex]= \frac{3}{4}[-\int_{-1}^0xdx + \int_0^1xdx-\int_{-1}^0x^3dx + \int_0^1x^3dx] [/itex]

    [itex]= \frac{3}{4}[-\frac{1}{2}x^2\Big|_{-1}^0 +\frac{1}{2}x^2\Big|_0^1-\frac{1}{4}x^4\Big|_{-1}^0 +\frac{1}{4}x^4\Big|_0^1] [/itex]

    = (3/4) [(1/2) + (1/2) + (1/4) + (1/4)]
    = (3/4)(3/2) = 9/8 = 1.125
     
  8. Oct 19, 2015 #7

    Ray Vickson

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    This cannot possibly be right: ##|X| \leq 1## for all non-zero probability values, so ##E|X| \leq 1## (and, in fact, ##E|X| < 1## strictly).
     
  9. Oct 19, 2015 #8
    in that case I am lost =[
     
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