31. Probability
important facts and formula
1.Experiment :An operation which can
produce some well-defined outcome is called an experiment
2.Random experiment: An experiment in which all
possible outcome are known and the exact out put cannot be predicted in advance
is called an random experiment
Eg of performing random experiment:
(i)rolling an unbiased dice
(ii)tossing a fair coin
(iii)drawing a card from a pack of
well shuffled card
(iv)picking up a ball of certain
color from a bag containing ball of different colors
Details:
(i)when we throw a coin. Then
either a head(h) or a tail (t) appears.
(ii)a
dice is a solid cube, having 6 faces ,marked 1,2,3,4,5,6 respectively when we
throw a die , the outcome is the number that appear on its top face .
(iii)a pack of cards has 52 cards
it has 13 cards of each suit ,namely spades,
clubs ,hearts and diamonds
Cards of spades and clubs are black cards
Cards of hearts and diamonds are red cards
There are 4 honors of each suit
These are aces ,king ,queen and jack
These are called face cards
3.Sample space :When we
perform an experiment ,then the set S of all
possible outcome is called the sample space
eg of sample space:
(i)in tossing a coin ,s={h,t}
(ii)if two coin are tossed ,then
s={hh,tt,ht,th}.
(iii)in rolling a die we
have,s={1,2,3,4,5,6}.
4.event:Any subset of a sample space.
5.Probability
of occurrence of an event.
let S be the sample space and E be
the event .
then,EÍS.
P(E)=n(E)/n(S).
6.Results on Probability:
(i)P(S) = 1 (ii)0<P(E)<1 (iii)P(f)=0
(iv)For any event a and b, we have:
P(aÈb)=P(a)+P(b)-P(aÈb)
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(v)If A denotes (not-a),then
P(A)=1-P(A).
SOLVED EXAMPLES
Ex 1. In a throw of a coin ,find the
probability of getting a head.
sol. Here s={h,t} and e={h}.
P(E)=n(E)/n(S)=1/2
Ex2.Two unbiased coin are tossed .what is the probability of
getting atmost one head?
sol.Here s={hh,ht,th,tt}
Let Ee=event of getting one head
e={tt,ht,th}
p(e)=n(e)/n(s)=3/4
Ex3.An unbiased die is tossed .find
the probability of getting a multiple of 3
sol. Here s={1,2,3,4,5,6}
Let e be the event of getting the multiple of 3
then ,e={3,6}
p(e)=n(e)/n(s)=2/6=1/3
ex4.
in a simultaneous throw of pair of dice .find the probability
of getting the total more than 7
sol.
Here n(s)=(6*6)=36
let e=event of getting a total more than 7
={(2,6),(3,5),(3,6),(4,4),(4,5),(4,6),(5,3),(5,4),(5,5),(5,6),(6,2),(6,3),(6,4),(6,5),(6,6)}
p(e)=n(e)/n(s)=15/36=5/12.
Ex5. A bag contains 6 white and 4
black balls .2 balls are drawn at random. find the probability that they are of
same colour.
Sol
.let s be the sample space
Then n(s)=no of ways of drawing 2 balls out of
(6+4)=10c2=(10*9)/(2*1)=45
Let e=event of getting both balls
of same colour
Then
n(e)=no of ways(2 balls out of
six) or(2 balls out of 4)
=(6c2+4c2)=(6*5)/(2*1)+(4*3)/(2*1)=15+6=21
p(e)=n(e)/n(s)=21/45=7/15
Ex6.Two dice are thrown together .What
is the probability that the sum of the number on the two faces is divided by 4
or 6
sol. Clearly n(s)=6*6=36
Let
E be the event that the sum of the numbers on the two faces is divided by
4 or 6.Then
e={(1,3),(1,5),(2,2),(2,4),(2,6),(3,1),(3,3),(3,5),(4,2),(4,4),(5,1),(5,3),(6,2),
(6,6)}
n(e)=14.
Hence p(e)=n(e)/n(s)=14/36=7/18
Ex7.Two cards are drawn at random from
a pack of 52 cards.what is the probability that either both are black or both are queen?
sol. We have
n(s)=52c2=(52*51)/(2*1)=1326.
Let A=event of getting both black
cards
B=event of getting both queens
aÇb=event of getting queen of black
cards
n(A)=26c2=(26*25)/(2*1)=325,
n(b)=4c2=(4*3)/(2*1)=6
and
n(aÇb)=2c2=1
p(A)=n(A)/n(S)=325/1326;
p(B)=n(B)/n(S)=6/1326 and
p(aÇb)=n(aÇb)/n(s)=1/1326
p(aÈb)=p(a)+p(b)-p(aÇb)=(325+6-1/1326)=330/1326=55/221
