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Math 151 Midterm 2 Solutions

1.

Given a random variable X with mean and variance 2 > 0. Construct a random variable f(X)with mean 1 and variance 2.

Solution.

We will use two identities:E[aX + b] = aE[X] + b for any real numbers a and bV ar(cX + d) = c2V ar(X) for any real numbers c and dFirst, we will transfrom the variance. Apply the above identities:

Then2

X has variance 2 and mean2

.Applying them again, mean changes and we get:

f(X) =2

X2

+ 1 has mean 1 and variance 2.

2.

For a nonnegative integer-valued random variable N, show that

i=0 iP[N > i] =12(E[N

2] E[N])

Solution.1

2

(E[N2] E[N]) =

i=0i2i2

P[N = i]Now, lets count number of P[N=i]-s for each i in the expression

i=0 iP[N > i]

This is exactly sum of js for each 1 j < i, because P[N > j] =

i=j+1 P[N = i]

Hence numbers of P[N = i]-s is 1 + 2 + ... + i 1 = (i1)i2 =i2i2 .

Since this is for each i, the two expressions are equal.

3.

X and Y are independent exponential random variables each having parameter . Let Z = X + Y,V = eX be random variables. Find the joint density function of Z and V.

Solution.

g1(x, y) = x + y, g2(x, y) = ex

Partial derivatives of g1 with respect to x and y are both 1 and partial derivatives of g2 with respectto x and y are ex and 0, respectively. Then |J| = ex. Page 300 of textbook gives:fZ,V(z, v) = fX,Y(x, y)|J|1 = 2e(x+y)ex = 2ezv1 when lnv > 0 and z lnv > 0,0, otherwise.Notice that final restrictions are direct consequence of the fact that expontial r.v is nonzero forx = lnv > 0 and y = z x = z lnv > 0

4.

The joint density of X and Y is given by f(x, y) = ey

y, 0 < x < y, 0 < y <

Compute E[X2|Y = y].

Solution.

fY(y) =y0

f(x, y)dx =y0

ey

y= ey

fX|Y(x|y) =f(x,y)f(y) =

1y

, 0 < x < y, 0 < y <

E[X2|Y = y] =y0

x2fX|Y(x|y)dx =y0

x2

ydx = 1

yx3

3 |yx=0 =

y2

3

5.

An airline company sells 200 tickets for a plane with 198 seats, knowing that the probability apassenger will not show up for the flight is 0.01. Use the Poisson approximation to compute theprobability they will have enough seats for all passengers who show up.

Solution.

In the language of Poisson approximation we have n = 200, p = 0.01Hence, = np = 2If N is the number of passenger who do not show up. We want:P(N 2) = 1 P[N = 0] P[N = 1] = 1 e21 e22 = 1 3e2

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6.

The time required to repair a machine is an exponential random variable with = 1 Find:(i) The probability that the repair time exceeds 3 hours.(ii) Given that its duration exceeds 8 hours, what is the conditional probability that a repair timetakes at least 10 hours.

Solution.(i) P[X > 3] =

3

exdx = e3

(ii) P[X > 10|X > 8] =P[(X>10)

T(X>8)]

P[X>8] =P[X>10]P[X>8] =

e10

e8= e2