Continuous Random Variable: Distribution Function & Expected Value












0












$begingroup$


Given



$$ F(x) = begin{cases}
0, & x < 1 \
c(x-1)^2, & 1 leq x < 2 \
1, & x geq 2
end{cases} $$




  1. Find the values of $c$ for which $F(x)$ is the distribution function of a random variable $X$.

  2. Establish for what value of $c$ the random variable $X$ is continuous, and in this case derive $E(X)$.


Solution





  1. $F(x)$ is the distribution function of a random variable $X$ for $0 leq c leq 1$.

  2. For $c=1$ the random variable $X$ is continuous and in this case it results $f(x) = 2(x-1)$ for $1 leq x < 2$ so it results in
    $$ E(X) = int_{1}^{2} 2x(x-1) , mathrm{d}x = 2 left[ frac{x^3}{3} - frac{x^2}{2}right]_{1}^{2} = frac{5}{3} $$


What is the proper way to solve this exercise?
I tried by using



$$ int_{-infty}^{infty} f(x) , mathrm{d}x = 1, $$



but it leads me to a single value of $c=3$, which is not the case of the solution. Is there any formula I'm missing?










share|cite|improve this question











$endgroup$

















    0












    $begingroup$


    Given



    $$ F(x) = begin{cases}
    0, & x < 1 \
    c(x-1)^2, & 1 leq x < 2 \
    1, & x geq 2
    end{cases} $$




    1. Find the values of $c$ for which $F(x)$ is the distribution function of a random variable $X$.

    2. Establish for what value of $c$ the random variable $X$ is continuous, and in this case derive $E(X)$.


    Solution





    1. $F(x)$ is the distribution function of a random variable $X$ for $0 leq c leq 1$.

    2. For $c=1$ the random variable $X$ is continuous and in this case it results $f(x) = 2(x-1)$ for $1 leq x < 2$ so it results in
      $$ E(X) = int_{1}^{2} 2x(x-1) , mathrm{d}x = 2 left[ frac{x^3}{3} - frac{x^2}{2}right]_{1}^{2} = frac{5}{3} $$


    What is the proper way to solve this exercise?
    I tried by using



    $$ int_{-infty}^{infty} f(x) , mathrm{d}x = 1, $$



    but it leads me to a single value of $c=3$, which is not the case of the solution. Is there any formula I'm missing?










    share|cite|improve this question











    $endgroup$















      0












      0








      0





      $begingroup$


      Given



      $$ F(x) = begin{cases}
      0, & x < 1 \
      c(x-1)^2, & 1 leq x < 2 \
      1, & x geq 2
      end{cases} $$




      1. Find the values of $c$ for which $F(x)$ is the distribution function of a random variable $X$.

      2. Establish for what value of $c$ the random variable $X$ is continuous, and in this case derive $E(X)$.


      Solution





      1. $F(x)$ is the distribution function of a random variable $X$ for $0 leq c leq 1$.

      2. For $c=1$ the random variable $X$ is continuous and in this case it results $f(x) = 2(x-1)$ for $1 leq x < 2$ so it results in
        $$ E(X) = int_{1}^{2} 2x(x-1) , mathrm{d}x = 2 left[ frac{x^3}{3} - frac{x^2}{2}right]_{1}^{2} = frac{5}{3} $$


      What is the proper way to solve this exercise?
      I tried by using



      $$ int_{-infty}^{infty} f(x) , mathrm{d}x = 1, $$



      but it leads me to a single value of $c=3$, which is not the case of the solution. Is there any formula I'm missing?










      share|cite|improve this question











      $endgroup$




      Given



      $$ F(x) = begin{cases}
      0, & x < 1 \
      c(x-1)^2, & 1 leq x < 2 \
      1, & x geq 2
      end{cases} $$




      1. Find the values of $c$ for which $F(x)$ is the distribution function of a random variable $X$.

      2. Establish for what value of $c$ the random variable $X$ is continuous, and in this case derive $E(X)$.


      Solution





      1. $F(x)$ is the distribution function of a random variable $X$ for $0 leq c leq 1$.

      2. For $c=1$ the random variable $X$ is continuous and in this case it results $f(x) = 2(x-1)$ for $1 leq x < 2$ so it results in
        $$ E(X) = int_{1}^{2} 2x(x-1) , mathrm{d}x = 2 left[ frac{x^3}{3} - frac{x^2}{2}right]_{1}^{2} = frac{5}{3} $$


      What is the proper way to solve this exercise?
      I tried by using



      $$ int_{-infty}^{infty} f(x) , mathrm{d}x = 1, $$



      but it leads me to a single value of $c=3$, which is not the case of the solution. Is there any formula I'm missing?







      probability-distributions random-variables expected-value






      share|cite|improve this question















      share|cite|improve this question













      share|cite|improve this question




      share|cite|improve this question








      edited Dec 30 '18 at 16:21









      Sangchul Lee

      94.8k12170276




      94.8k12170276










      asked Dec 30 '18 at 15:58









      1Z101Z10

      1105




      1105






















          2 Answers
          2






          active

          oldest

          votes


















          1












          $begingroup$



          1. Recall that $F$ is a distribution function (a.k.a. c.d.f.) if and only if the following properties hold:






            • $F$ is non-decreasing,


            • $F(x) in [0, 1]$ for all $x in mathbb{R}$,


            • $F$ is right-continuous.




            The $F$ defined as in the problem is always right-continuous, so the last property is not our concern. For the rest, a moment of thought tells that we must have $0 leq lim_{xto 2^-} F(x) leq 1$, which is equivalent to $0 leq c leq 1$.



            $hspace{2em}$ Comparison for different values of c




          2. $X$ is continuous if and only if $F$ is absolutely continuous, which means that



            $$F(x) = int_{-infty}^{x} f(t) , mathrm{d}t$$



            for some non-negative function $f$ and for all $x in mathbb{R}$. A necessary condition is that $F$ itself is continuous. (And practically, this is also a sufficient condition at the level of preliminary probability course, as one never deals with singular distribution at this level.)



            Now, we see that $c = 1$ is the only choice for which $F$ is continuous. And with this choice, we easily identify the p.d.f. $f$ as



            $$ f(x) = begin{cases} 2(x-1), & 1 < x < 2 \ 0, & text{otherwise} end{cases}. $$



            Plugging this to the formula $E(X) = int_{-infty}^{infty} xf(x) , mathrm{d}x$ gives the desired answer.








          share|cite|improve this answer











          $endgroup$













          • $begingroup$
            According to Wikipedia it is enough if $F$ is continuous.
            $endgroup$
            – drhab
            Dec 30 '18 at 16:37










          • $begingroup$
            @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:44












          • $begingroup$
            Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
            $endgroup$
            – 1Z10
            Dec 30 '18 at 16:46






          • 1




            $begingroup$
            @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:48








          • 1




            $begingroup$
            @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:58



















          1












          $begingroup$

          By solving this the PDF should be left out.



          The question is about a CDF which is a function with special properties.



          In 1) it is asked: for what $c$ does this function indeed have these properties?



          In 2) it must be noted that a random variable is continuous iff its CDF is continuous.



          So actually it is asked: for what $c$ found in 1) is the function continuous?






          share|cite|improve this answer









          $endgroup$













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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes









            1












            $begingroup$



            1. Recall that $F$ is a distribution function (a.k.a. c.d.f.) if and only if the following properties hold:






              • $F$ is non-decreasing,


              • $F(x) in [0, 1]$ for all $x in mathbb{R}$,


              • $F$ is right-continuous.




              The $F$ defined as in the problem is always right-continuous, so the last property is not our concern. For the rest, a moment of thought tells that we must have $0 leq lim_{xto 2^-} F(x) leq 1$, which is equivalent to $0 leq c leq 1$.



              $hspace{2em}$ Comparison for different values of c




            2. $X$ is continuous if and only if $F$ is absolutely continuous, which means that



              $$F(x) = int_{-infty}^{x} f(t) , mathrm{d}t$$



              for some non-negative function $f$ and for all $x in mathbb{R}$. A necessary condition is that $F$ itself is continuous. (And practically, this is also a sufficient condition at the level of preliminary probability course, as one never deals with singular distribution at this level.)



              Now, we see that $c = 1$ is the only choice for which $F$ is continuous. And with this choice, we easily identify the p.d.f. $f$ as



              $$ f(x) = begin{cases} 2(x-1), & 1 < x < 2 \ 0, & text{otherwise} end{cases}. $$



              Plugging this to the formula $E(X) = int_{-infty}^{infty} xf(x) , mathrm{d}x$ gives the desired answer.








            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              According to Wikipedia it is enough if $F$ is continuous.
              $endgroup$
              – drhab
              Dec 30 '18 at 16:37










            • $begingroup$
              @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:44












            • $begingroup$
              Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
              $endgroup$
              – 1Z10
              Dec 30 '18 at 16:46






            • 1




              $begingroup$
              @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:48








            • 1




              $begingroup$
              @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:58
















            1












            $begingroup$



            1. Recall that $F$ is a distribution function (a.k.a. c.d.f.) if and only if the following properties hold:






              • $F$ is non-decreasing,


              • $F(x) in [0, 1]$ for all $x in mathbb{R}$,


              • $F$ is right-continuous.




              The $F$ defined as in the problem is always right-continuous, so the last property is not our concern. For the rest, a moment of thought tells that we must have $0 leq lim_{xto 2^-} F(x) leq 1$, which is equivalent to $0 leq c leq 1$.



              $hspace{2em}$ Comparison for different values of c




            2. $X$ is continuous if and only if $F$ is absolutely continuous, which means that



              $$F(x) = int_{-infty}^{x} f(t) , mathrm{d}t$$



              for some non-negative function $f$ and for all $x in mathbb{R}$. A necessary condition is that $F$ itself is continuous. (And practically, this is also a sufficient condition at the level of preliminary probability course, as one never deals with singular distribution at this level.)



              Now, we see that $c = 1$ is the only choice for which $F$ is continuous. And with this choice, we easily identify the p.d.f. $f$ as



              $$ f(x) = begin{cases} 2(x-1), & 1 < x < 2 \ 0, & text{otherwise} end{cases}. $$



              Plugging this to the formula $E(X) = int_{-infty}^{infty} xf(x) , mathrm{d}x$ gives the desired answer.








            share|cite|improve this answer











            $endgroup$













            • $begingroup$
              According to Wikipedia it is enough if $F$ is continuous.
              $endgroup$
              – drhab
              Dec 30 '18 at 16:37










            • $begingroup$
              @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:44












            • $begingroup$
              Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
              $endgroup$
              – 1Z10
              Dec 30 '18 at 16:46






            • 1




              $begingroup$
              @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:48








            • 1




              $begingroup$
              @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:58














            1












            1








            1





            $begingroup$



            1. Recall that $F$ is a distribution function (a.k.a. c.d.f.) if and only if the following properties hold:






              • $F$ is non-decreasing,


              • $F(x) in [0, 1]$ for all $x in mathbb{R}$,


              • $F$ is right-continuous.




              The $F$ defined as in the problem is always right-continuous, so the last property is not our concern. For the rest, a moment of thought tells that we must have $0 leq lim_{xto 2^-} F(x) leq 1$, which is equivalent to $0 leq c leq 1$.



              $hspace{2em}$ Comparison for different values of c




            2. $X$ is continuous if and only if $F$ is absolutely continuous, which means that



              $$F(x) = int_{-infty}^{x} f(t) , mathrm{d}t$$



              for some non-negative function $f$ and for all $x in mathbb{R}$. A necessary condition is that $F$ itself is continuous. (And practically, this is also a sufficient condition at the level of preliminary probability course, as one never deals with singular distribution at this level.)



              Now, we see that $c = 1$ is the only choice for which $F$ is continuous. And with this choice, we easily identify the p.d.f. $f$ as



              $$ f(x) = begin{cases} 2(x-1), & 1 < x < 2 \ 0, & text{otherwise} end{cases}. $$



              Plugging this to the formula $E(X) = int_{-infty}^{infty} xf(x) , mathrm{d}x$ gives the desired answer.








            share|cite|improve this answer











            $endgroup$





            1. Recall that $F$ is a distribution function (a.k.a. c.d.f.) if and only if the following properties hold:






              • $F$ is non-decreasing,


              • $F(x) in [0, 1]$ for all $x in mathbb{R}$,


              • $F$ is right-continuous.




              The $F$ defined as in the problem is always right-continuous, so the last property is not our concern. For the rest, a moment of thought tells that we must have $0 leq lim_{xto 2^-} F(x) leq 1$, which is equivalent to $0 leq c leq 1$.



              $hspace{2em}$ Comparison for different values of c




            2. $X$ is continuous if and only if $F$ is absolutely continuous, which means that



              $$F(x) = int_{-infty}^{x} f(t) , mathrm{d}t$$



              for some non-negative function $f$ and for all $x in mathbb{R}$. A necessary condition is that $F$ itself is continuous. (And practically, this is also a sufficient condition at the level of preliminary probability course, as one never deals with singular distribution at this level.)



              Now, we see that $c = 1$ is the only choice for which $F$ is continuous. And with this choice, we easily identify the p.d.f. $f$ as



              $$ f(x) = begin{cases} 2(x-1), & 1 < x < 2 \ 0, & text{otherwise} end{cases}. $$



              Plugging this to the formula $E(X) = int_{-infty}^{infty} xf(x) , mathrm{d}x$ gives the desired answer.









            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited Dec 30 '18 at 16:57

























            answered Dec 30 '18 at 16:31









            Sangchul LeeSangchul Lee

            94.8k12170276




            94.8k12170276












            • $begingroup$
              According to Wikipedia it is enough if $F$ is continuous.
              $endgroup$
              – drhab
              Dec 30 '18 at 16:37










            • $begingroup$
              @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:44












            • $begingroup$
              Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
              $endgroup$
              – 1Z10
              Dec 30 '18 at 16:46






            • 1




              $begingroup$
              @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:48








            • 1




              $begingroup$
              @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:58


















            • $begingroup$
              According to Wikipedia it is enough if $F$ is continuous.
              $endgroup$
              – drhab
              Dec 30 '18 at 16:37










            • $begingroup$
              @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:44












            • $begingroup$
              Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
              $endgroup$
              – 1Z10
              Dec 30 '18 at 16:46






            • 1




              $begingroup$
              @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:48








            • 1




              $begingroup$
              @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
              $endgroup$
              – Sangchul Lee
              Dec 30 '18 at 16:58
















            $begingroup$
            According to Wikipedia it is enough if $F$ is continuous.
            $endgroup$
            – drhab
            Dec 30 '18 at 16:37




            $begingroup$
            According to Wikipedia it is enough if $F$ is continuous.
            $endgroup$
            – drhab
            Dec 30 '18 at 16:37












            $begingroup$
            @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:44






            $begingroup$
            @drhab, Perhaps there are different conventions for what it means to be a continuous distribution. But in most literature that I have checked, the definition of continuous distribution does not encompass singular distributions. (See this MIT open course, for instance.)
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:44














            $begingroup$
            Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
            $endgroup$
            – 1Z10
            Dec 30 '18 at 16:46




            $begingroup$
            Could you please explain further how do you conclude that c=1 is the only choice for which F is continuous?
            $endgroup$
            – 1Z10
            Dec 30 '18 at 16:46




            1




            1




            $begingroup$
            @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:48






            $begingroup$
            @1Z10, Plotting the graph of $F$ will be convincing enough. But if one needs a solid proof, notice that $x = 2$ is the only possible discontinuity of $F$. Since $F(2) = 1= lim_{xto 2^+}F(x)$ and $lim_{xto2^-}F(x) = c$, it suffices to show that they match. So we obtain the necessary condition $c = 1$.
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:48






            1




            1




            $begingroup$
            @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:58




            $begingroup$
            @1Z10, Glad it helped. I also added plots of $F$ for different values of $c$. :)
            $endgroup$
            – Sangchul Lee
            Dec 30 '18 at 16:58











            1












            $begingroup$

            By solving this the PDF should be left out.



            The question is about a CDF which is a function with special properties.



            In 1) it is asked: for what $c$ does this function indeed have these properties?



            In 2) it must be noted that a random variable is continuous iff its CDF is continuous.



            So actually it is asked: for what $c$ found in 1) is the function continuous?






            share|cite|improve this answer









            $endgroup$


















              1












              $begingroup$

              By solving this the PDF should be left out.



              The question is about a CDF which is a function with special properties.



              In 1) it is asked: for what $c$ does this function indeed have these properties?



              In 2) it must be noted that a random variable is continuous iff its CDF is continuous.



              So actually it is asked: for what $c$ found in 1) is the function continuous?






              share|cite|improve this answer









              $endgroup$
















                1












                1








                1





                $begingroup$

                By solving this the PDF should be left out.



                The question is about a CDF which is a function with special properties.



                In 1) it is asked: for what $c$ does this function indeed have these properties?



                In 2) it must be noted that a random variable is continuous iff its CDF is continuous.



                So actually it is asked: for what $c$ found in 1) is the function continuous?






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                $endgroup$



                By solving this the PDF should be left out.



                The question is about a CDF which is a function with special properties.



                In 1) it is asked: for what $c$ does this function indeed have these properties?



                In 2) it must be noted that a random variable is continuous iff its CDF is continuous.



                So actually it is asked: for what $c$ found in 1) is the function continuous?







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered Dec 30 '18 at 16:17









                drhabdrhab

                102k545136




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