MATH20701 2020
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MATH20701
1. Given a probability density function
fX,Y (x,y) =
then:
(a) show that A = 2; [3 marks]
(b) find the marginal probability density function of X; [4 marks]
(c) find the marginal probability density function of Y ; [4 marks]
(d) find the conditional probability density function of Y given X; [4 marks]
(e) find P(X + Y > 3 | X = 1); [5 marks]
2. Let X and Y be independent random variables each with the same exponential distribution: X ⇠ Y ⇠ Exp(λ) [remember that the cumulative distribution function of the exponential distribution is given by FX (x) = 1 − e −λx for λ,x > 0].
(a) Let U = X + Y . Show that the probability density function for U is given by
fU (u) = [5 marks]
(b) Let V = X − Y . Show that the cumulative distribution function for V is given by
FV (v) = ( [7 marks]
(c) Let Z = X/Y . Define u = y and z = x/y and write down the determinant of the Jacobian of this bivariate transform. [4 marks]
(d) Given
fU,Z (u,z) = fX,Y (uz,u)| − u|
for (u,v) such that fX,Y (uz,u) > 0 and zero elsewhere show that the probability density function
for Z is given by
fZ (z) = (
for z > 0
z < 0 [4 marks]
3. Let X be a discrete random variable with probability mass function given by
c
P(X = k) =
for k = 0, 1, 2, 3, . . . , and 0 otherwise, where c > 0. Let Y ⇠ Po(λ) be a Poisson random variable with probability mass function given by
λm e −λ
m!
for m = 0, 1, 2, . . . and 0 otherwise.
(a) Show that c = e −1 . [3 marks]
(b) Assuming X and Y are independent write down the joint probability mass function. [3 marks]
(c) Find P(X + Y = 2). [4 marks]
(d) Let a random variable Z = X + Y . By finding the probability mass function prove that X + Y ⇠ Po(λ +1). [5 marks]
(e) Derive the conditional probability mass functions for X = k given Z = n (for k < n). [5 marks]
4. The joint probability density function of random variables X and Y is
fX,Y (x,y) = {
−1 < x < 1, x2 < y < 1;
elsewhere.
(i) Show that, for −1 < x < 1, the conditional density function of Y given X = x is fY |X (y | x) = [5 marks]
(ii) For −1 < x < 1, show that
E[Y | X = x] = (1 + x2 ). [5 marks]
(iii) Hence, or otherwise, determine E[E[Y | X]]. [5 marks]
(iv) For −1 < x < 1, calculate the conditional moment generating function (m.g.f.) of Y given X = x. By identifying the coeicient of t in the power series expansion of the m.g.f., verify that E[Y | X = x] = (1 + x2 ). [5 marks]
5. Let W be a random variable with probability mass function (p.m.f.)
P(W = k) = k = 0, 1, ...;
where λ is a positive constant.
(i) Show that the probability generating function of W is
PW (t) = exp{λ(t − 1)} − 1 < t < 1. [5 marks]
(ii) Let X1 ,X2 ... be independent random variables, each with p.m.f.
P(X = k) = pqk−1 k = 1, 2 ....
where p and q are positive constants such that p + q = 1.
Show that the moment generating function (m.g.f) of X is
pet
1 − qet [5 marks]
(iii) Let Z = P1<i<W Xi with W as defined in part (i), X1 ,X2 ... as defined in part(ii) and Z = 0 when W = 0. Given that W,X1 ,X2 , . . . are independent, show that the m.g.f. of Z is
MZ (t) = exp { } −1 < t < − log q . [5 marks]
(iv) Hence, or otherwise, determine the mean of Z . [5 marks]
6. a) Given n ≥ 1 and independent random variables X1 ,X2 . . . ,Xn , each with cumulative distribution function (c.d.f) F(x):
(i) derive the c.d.f. of
Y = min{X1 ,X2 . . . ,Xn }; [5 marks]
(ii) determine the c.d.f. of Y when X1 has probability density function
f(x) = { λe0(−)λx
where λ is a positive constant.
0 < x < 1;
elsewhere; [5 marks]
b)
(i) State the Central Limit Theorem. [3 marks]
(ii) The number of hits per minute on a certain website can be modelled by a Poisson random variable with probability mass function
P(X = k) =
k = 0, 1, ....
Assuming that the numbers of hits in disjoint time intervals are independent, use the Central Limit Theorem to find the approximate probability of the number of hits in a 4 hour period belonging to
the interval [450, 500]. Carefully justify each step in your argument. [7 marks]
2023-01-09