** Formulae **

P(E)= 1−P(E′)

P(ϕ)= 0

If A⊆B ,then P(A)≤P(B)

For any event E, 0≤P(E)≤1

For A and B, P(A∪B)= P(A)+P(B)−P(A∩B)

P(A∪B∪C)= P(A)+P(B)+P(C)−P(AB)−P(AC)−P(BC)+P(ABC)

P(A∩B)= P(A)*P(B|A)

P(A∩B∩C)= P(A)*P(B|A)*P(C|A∩B)

(Bayes’ Theorem) P(A|B)=

Let S be the same space, then the probability of occurrence of an event E is denoted by P(E) and is defined as

**Sample Space :**

In tossing a coin, S = {H, T}

If two coins are tossed, the S = {HH, HT, TH, TT}

In rolling a dice, we have, S = {1, 2, 3, 4, 5, 6}

**Example:** Two coins are tossed, find the probability that two heads are obtained.

**Explanation:**The sample space S is given by.

S = {(H,T),(H,H),(T,H),(T,T)}

Let E be the event “two heads are obtained”. ‘

E = {(H,H)}

We use the formula of the classical probability.

P(E) = n(E) / n(S) = 1 / 4

Hence the probability of two heads obtained is ¼.