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

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29.14 The normal distribution 963

Solution

Themeantimebetweenbreakdowns = 17 = 1 α ,soα= 1 .Thusthep.d.f., f (t),is

17

given by

f(t)= 1 17 e−t/17 t 0

We require the probability thatthe machine breaks down ina15 day period:

∫ 15

P(0t15)= f(t)dt

0

∫ 15

1

=

0 17 e−t/17 dt

] 15

=

[−e −t/17

0

= −e −15/17 +1

= 0.5862

There isa58.62% chance thatthe machine will break down ina15 day period.

EXERCISES29.13

1 Aservice engineer receives onaveragean emergency

callevery 3 hours. If the time betweencalls follows

an exponential distribution, calculatethe probability

thatthe time from oneemergency callto the next is

(a) greaterthan3hours

(b) less than4.5hours

2 Themeantimebetweenbreakdownsforacertaintype

ofmachineis400hours.Calculatetheprobabilitythat

the time between breakdowns foraparticular

machine is

(a) greater than450 hours

(b) less than350 hours

3 The meantime taken byan engineerto repair an

electrical faultin asystem is 2.7hours.Calculate the

probability thatthe engineerwillrepair afaultin less

thanthe meantime.

Solutions

1 (a) 0.3679 (b) 0.7769

3 0.6321

2 (a) 0.3247 (b) 0.5831

29.14 THENORMALDISTRIBUTION

The normal probability density function, commonly called the normal distribution, is

one of the most important and widely used. It is used to calculate the probable values

of continuous variables, for example weight, length, density, error measurement. Probabilities

calculated using the normal distribution have been shown to reflect accurately

thosewhichwould befound usingactual data.

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