Page 33 - Math SL HB Sem 3

C

{, Note the two key facts about probability:

'1 . Probability of an event must not exceed 1 or be smallar than 0:

0< P< 1

where P = 0 means that the event is impossible to happen while P = 1 means that the event
is certain to happen.

2. Thesumof all the probabilities of all the random samples must equal toone:

n
T I
L P I

i

Relationships between events:
Usually, we want to consider two or more events and see how they (and their probabilities) are related to
one another. Let's first remind ourselves of the two types of combinations of events:

) lntersection (AND)

A
^B

This represents both event A and B are occurring. On a Venn diagram, it is the overlap region
(shaded) of set .4 and set B.

1A B
t
T I
t
T
I
t t
U

As seen from the Venn diagram, there is only one point in the shaded region.
Hence,

n(AnB), 1
P(A i B\ - ---:-----------: = -12
n(U)

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