Introduction to Probability and Some basic definitions.

Let's start with the basics:

What is the probability ?

Informally, the probability is all about representing your chances of success or failure with the help of mathematics.  It is a mathematical measure of degree randomness.

Some important terms:

Random Experiment:-

When we perform any task, we actually know all the outcomes but we don't know what outcome will come in advance. for example: 

When we toss an unbiased coin, we know either head or tail will show up but we don't know what comes when? This is called a random experiment.


Sample Space: Set of all possible outcomes of an experiment. Example:

1.When you toss a coin 

Sample Space will be {head, tail}  

2. When you roll a dice

SS= {1,2,3,4,5,6}

Note: Sample space is a set. It means you can't repeat things inside a sample space.


Events: Subset of sample space is called Events. Example:

1. When you toss a coin getting ahead is or getting a tail is a subset of Sample Space.

2. On rolling a dice "getting even no " is an event.


Mutually Exclusive events:

Two Events E1 and E2 are said to be mutually exclusive if 

E1 ∩ E2 = Ø


Independent Events: Two events are said to be independent if happening of one event doesn't affect another event. Example:

When you toss 2 coins getting head on the first coin is not going to affect getting anything on the second coin. So Both are independent events.


Some Rules:-

For each event E in a sample Space S, we assign a number P(E) Such that

1. 0 <= P(E) <= 1

2. P(S)= 1 = P(E) + P(Ec)

3. For any sequence of events E1, E2 ......En  that are mutually events then

P(E1∪ E2 ∪ E3 ∪...... ∪ En) = P(E1) + P(E2) + P(E3) +......... P(En)


Probability of happening of an event E is=

Total number of favorable outcome  / Total number of possible outcomes


P(E) = favourable outcome  / possible outcomes

  • Probability of an impossible event is always O
  • Probability of a certain event is always 1.



1.On tossing a coin find the probability of getting a head.


When you toss a coin Possible sample space is 

S= {H, T}

P(getting a head ) = 1 / 2

P (getting a tail )    = 1 /2