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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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948 Chapter 29 Statistics and probability distributions

EXERCISES29.8

1 Ap.d.f.h(x)ofthe random variablexisdefined by

h(x) = 3 4 (1−x2 )

−1x1

(a) Calculate the expected value ofx.

(b) Calculate the standard deviation ofx.

2 Arandom variable,t,has p.d.f.H(t)given by

H(t)=3e −3t

t0

(a) Calculate the expected value oft.

(b) Calculate the standard deviation oft.

3 Adiscrete random variable, w,has a known

probability distribution

w −1 −0.5 0 0.5 1

P(w) 0.1 0.17 0.4 0.21 0.12

Calculate the standard deviation of w.

4 Adiscrete randomvariable,y,has a probability

distribution

y 6 7 8 9 10 11

P(y) 0.13 0.26 0.14 0.09 0.11 0.27

Calculate the standard deviation ofy.

5 Acontinuous random variablehas p.d.f.

{ x+1 −1x0

f(x)=

−x+1 0<x1

(a) Calculate the expected value ofx.

(b) Calculate the standard deviation ofx.

Solutions

1 (a) 0 (b) 0.4472

4 1.84

2 (a)

3 0.5598

1

3 (b)

1

3

5 (a) 0 (b) 0.4082

29.9 PERMUTATIONSANDCOMBINATIONS

Bothpermutationsandcombinationsareusedextensivelyinthecalculationofprobabilities.

29.9.1 Permutations

Thefollowing problem introducespermutations.

Engineeringapplication29.2

Linkingprocesscontrolcomputers

A primary and a secondary route must be chosen from available routes A, B and C,

for the linking of two process control computers in order to provide redundancy in

case one fails.Inhow many ways can the choicesbemade?

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