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+9 votes

A point is randomly selected with uniform probability in the $X-Y$ plane within the rectangle with corners at $(0,0), (1,0), (1,2)$ and $(0,2).$ If $p$ is the length of the position vector of the point, the expected value of $p^{2}$ is

  1. $\left(\dfrac{2}{3}\right)$
  2. $\quad 1$
  3. $\left(\dfrac{4}{3}\right)$
  4. $\left(\dfrac{5}{3}\right)$
asked in Probability by Veteran (69k points)
edited by | 1.4k views
I have one doubt in the posted solutions. We have one more ways to calculate probability of each point in 2-D plane  $\rightarrow$
For ex. P = $\frac{1}{2}$  will come via two points $\left ( \frac{1}{2}, 0 \right ) and \left (0, \frac{1}{2} \right )$ respectively. Now probability of obtaining each point is $\frac{1}{2}*1$ and $\frac{1}{2}*1$ (because X and Y both are following uniform distribution and both are independent). So total probability of getting P =  $\frac{1}{2}$ is $\left ( \frac{1}{2} + \frac{1}{2} \right )$ . But via mentioned method probability for P =  $\frac{1}{2}$ will be $\frac{1}{\sqrt{5}}$ .

Where am i doing mistake ?

First of all considering uniform random variable which is continuous the probability at a point is 0.So, you cannot solve like this.

Thank you @VS ji,

But first of all my following assumption is wrong ->

For ex. P = 1/2  will come via two points (1/2,0)and(0,1/2) respectively.

P = 1/2 will come via all the points in quarter circle whose radius is 1/2. But if we will do via this approach then we will not be able to cover all points in mentioned rectangle. 

This one ...



2 Answers

+16 votes
Best answer

answer = option D

answered by Veteran (31k points)
selected by
Plz tell once again how u calculated the value of P(p) ?

here, $f(x) = P(p)$

why P(p) is 1/root (5)???
+16 votes
answered by Veteran (11.2k points)

2 doubt-

1. why we are taking P(p)dp while calculating E(p^2), why not P(p^2)dp  ?? 

2. why can't we first calculate expected value of P, and then take square of it. ??



It is asked to calculate expectation of p^2, i.e varience of p

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