Exponential distribution problem

toothpaste666
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Homework Statement


The amount of time that a surveillance camera will run without having to be reset is a random variable having the exponential distribution with beta = 50 days. find the probabilities that such a camera will
a) have to be reset in less than 20 days
b) not have to be reset in at least 60 days

The Attempt at a Solution


first we integrate the function
f(x) = (1/beta)e^(-x/beta) = (1/50)e^(-x/50)
after integrating we get
-e^(-x/50) evaluated from a to b
for part a)
we are evaluating from 0 to 20 so we get
-(e^(-20/50) - 1) = 1 - e^(-20/50) = .3297

for part b) we evaluate from 60 to ∞
-(0 - e^(-60/50)) = e^(-60/50) = .3012

is this method correct?
 
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It looks right to me.
 
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Likes toothpaste666
It looks OK to me as well.
Another way to do part b is this: ##Pr(x \ge 60) = 1 - Pr(0 \le x < 60)##
So ##\frac 1 {50}\int_{60} ^{\infty} e^{-x/50}dx = \frac 1 {50}\left(1 - \int_0^{60}e^{-x/50}dx\right)##
 
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Likes BHArat01 and toothpaste666
ok great thank you!
 
toothpaste666 said:

Homework Statement


The amount of time that a surveillance camera will run without having to be reset is a random variable having the exponential distribution with beta = 50 days. find the probabilities that such a camera will
a) have to be reset in less than 20 days
b) not have to be reset in at least 60 days

The Attempt at a Solution


first we integrate the function
f(x) = (1/beta)e^(-x/beta) = (1/50)e^(-x/50)
after integrating we get
-e^(-x/50) evaluated from a to b
for part a)
we are evaluating from 0 to 20 so we get
-(e^(-20/50) - 1) = 1 - e^(-20/50) = .3297

for part b) we evaluate from 60 to ∞
-(0 - e^(-60/50)) = e^(-60/50) = .3012

is this method correct?

Yes.

Here are a small number of handy facts about the exponential distribution---worth committing to memory (and pretty easy to derive for yourself).

If ##X## has distribution ##\text{Exp}(\lambda)## (so that its pdf on ##\{ x > 0 \}## is ##\lambda e^{-\lambda x}##) then:
\begin{array}{cl}<br /> 1)&amp; EX = \displaystyle \frac{1}{\lambda} \\<br /> 2)&amp;\text{Var} X = \displaystyle \frac{1}{\lambda^2} \\<br /> 3)&amp; \text{Coefficient of variation} \equiv \displaystyle \frac{\text{standard deviation}}{\text{mean}} = 1 \\<br /> 4) &amp; P(X &gt; x) = e^{- \lambda x} , \; x \geq 0 <br /> \end{array}<br />
From 4) it follows that ##P(X \leq x) = 1 - e^{- \lambda x}## for ##x \geq 0##. Also, remembering 3) is a convenient way of remembering 2).
 
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Likes andrewkirk
Can anyone tell me that what happens if there is no, not used in the b part of the question?
shortly, what is the answer if the b part is: have to be reset in at least 60 days
 
Talha said:
Can anyone tell me that what happens if there is no, not used in the b part of the question?
shortly, what is the answer if the b part is: have to be reset in at least 60 days

What do YOU think the answer should be? You have all the formulas needed to calculate the result.
 
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