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26. For a Poisson random variable N with mean ! find<br />

lim E( N N # 1).<br />

!"0<br />

!"0<br />

n=1<br />

!"0<br />

n=1<br />

PRACTICE EXAMINATION NO. 6<br />

A. !<br />

Solution.<br />

B. 0 C. 1 D. e E. Cannot be determined<br />

n<br />

!<br />

Let fN ( n)<br />

=<br />

n! " e#! for n = 0,1,2,… be the probability function of this random variable. We<br />

have<br />

+%<br />

+% fN ( n)<br />

lim E( N N # 1)<br />

= lim&<br />

n $ f n N # 1<br />

!"0<br />

N N #1 ( ) = lim&<br />

n $ =<br />

Pr N # 1<br />

Answer C.<br />

= lim<br />

+%<br />

&<br />

!"0<br />

n=1<br />

( )<br />

( )<br />

fN n<br />

n $<br />

1' fN 0<br />

1<br />

= lim<br />

!"0 1' fN 0<br />

!<br />

= lim<br />

!"0 1' e '!<br />

( )<br />

+%<br />

&<br />

n=0<br />

=<br />

1<br />

= lim<br />

!"0 1 ' fN 0<br />

n $ f N n<br />

lim<br />

de l'Hospital !"0<br />

( ) =<br />

( )<br />

n $ fN ( n)<br />

=<br />

( ) n=1<br />

E( N )<br />

1 ' fN ( 0)<br />

=<br />

ASM Study Manual for Course P/1 Actuarial Examination. © Copyright 2004-2007 by Krzysztof Ostaszewski - 281 -<br />

lim<br />

!"0<br />

1<br />

= <strong>1.</strong> '!<br />

e<br />

27. X is a normal random variable with mean zero and variance 1<br />

and Y is distributed<br />

2<br />

exponentially with mean <strong>1.</strong> X and Y are independent. Find the probability Pr Y > X 2 ( ).<br />

1 e<br />

A. B. C.<br />

e<br />

!<br />

Solution.<br />

We know that<br />

1<br />

fX ( x)<br />

=<br />

1<br />

2 2!<br />

e<br />

for !" < x < +", and<br />

! y<br />

fY ( y)<br />

= e<br />

for y > 0, so that<br />

" 1<br />

#<br />

2<br />

&<br />

%<br />

%<br />

2 %<br />

$<br />

%<br />

x (<br />

(<br />

1 (<br />

2 '<br />

(<br />

fX,Y ( x, y)<br />

= 1<br />

! e" x2 " y<br />

e<br />

+%<br />

&<br />

1 1 2<br />

D. E.<br />

2!<br />

2<br />

2<br />

= 1<br />

! e" x2

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