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914 Chapter 28 Probability

TheVenndiagramcorrespondingtothissituationisidenticaltothatinFigure28.1where

the setAcorresponds tothe eventA, and the eventBisrepresented byA.

Two events, A and B, are complementary if they are mutually exclusive and in a

singletrialeitherAorBmusthappen.

Hence,ifAandBarecomplementary then

and

P(A) +P(B) =1

P(A∩B)=0

Itis usualtodenotecomplementary events asAandA. For example,

A : the component istop quality

A : the component isnot topquality

B:n>6

B:n6

C : the circuithas failed tomeet the specification

C : the circuithas met the specification

Recall from Engineering application 28.3 that D was the event the component lasts

3 years or less, and thishad probabilityP(D) = 0.08. We could say

D : the component lasts 3 years orless

D : the component lasts more than3years

DandDarecomplementary events and so

P(D) +P(D)=1

P(D)=1−P(D)=1−0.08=0.92

Engineeringapplication28.5

Testingmultipleelectroniccomponents

Three transistors are tested. The probability that none of them works is 0.03. What

isthe probability thatatleastone transistor works?

Solution

We definethe eventsAandA:

A : all threetransistors fail

A : not all three transistorsfail

AandAare complementary events and so

P(A) = 0.03 P(A) =1−P(A) =0.97

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