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Mathematical Methods for Physics and Engineering - Matematica.NET

Mathematical Methods for Physics and Engineering - Matematica.NET

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30.9 IMPORTANT CONTINUOUS DISTRIBUTIONSexpression <strong>for</strong> the PDF of this Z does exist, but it is a rather complicatedcombination of exponentials <strong>and</strong> a modified Bessel function. §◮Two types of e-mail arrive independently <strong>and</strong> at r<strong>and</strong>om: external e-mails at a mean rateof one every five minutes <strong>and</strong> internal e-mails at a rate of two every five minutes. Calculatethe probability of receiving two or more e-mails in any two-minute interval.LetX = number of external e-mails per two-minute interval,Y = number of internal e-mails per two-minute interval.Since we expect on average one external e-mail <strong>and</strong> two internal e-mails every five minuteswe have X ∼ Po(0.4) <strong>and</strong> Y ∼ Po(0.8). Letting Z = X + Y we have Z ∼ Po(0.4+0.8) =Po(1.2). NowPr(Z ≥ 2) = 1 − Pr(Z

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