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Simple sample variance problem 
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#1
Jul709, 09:08 PM

P: 54

The length of time, in minutes, for an airplane to obtain clearance for takeoff is a random variable Y = 4X + 2, where X has the density function:
f(x) = 1/3 e^(x/3) for x > 0 Find the variance of Y.  I found the expected value of X to be 14 minutes, which was correct. This means the variance for X will be: E(X^2)  E(X) = 18  14 = 4. Will the following work for the variance? Var(Y) = (4^2)(4) = 64. If so, what does that mean to have a "negative" take off time? Thanks for your help!! :) 


#2
Jul809, 06:09 AM

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P: 39,564

The variance is NOT "E(X^2) E(x)" it is E(X^2) (E(x))^2.



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