Probability Class 11 Notes - Chapter 16

Probability is an integral part of class 11 maths syllabus and is not only important for class 11 exams but also for different engineering exams like JEE. In earlier classes, you may have studied the concept of probability as a measure of uncertainty of various phenomenon. Here, a brief introduction to probability is given based on the class 11 maths syllabus, which will help to learn the related concepts quickly and also to score good marks in the exam. Also, learn probability and statistics here.

The topics and subtopics covered in Class 11 Probability are listed below:

• Random Experiments
• Outcomes and sample space
• Event
• Occurrence of an event
• Types of events
• Complementary Event
• Mutually exclusive events
• Exhaustive events
• Axiomatic Approach to Probability
• Probability of an event
• Probabilities of equally likely outcomes

For More Information On Probability, Watch The Below Video.

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Probability for Class 11: Key Concepts

• An experiment is said to be a random experiment if there are more than one possible outcomes and it is impossible to predict the outcome in advance.
• All possible results of an experiment are called its outcomes.
• Let us consider an experiment of rolling a die. All possible outcomes are 6, 5, 4, 3, 2, or 1. The set of all these outcomes {6, 5, 4, 3, 2, 1} is known as the sample space and is denoted by ‘S’.
• Let us consider an experiment of tossing Two coins once. Since the coin can turn up Tail or Head, therefore, all the possible outcomes are:
  Both coins – Head = HH, Both coins – Tail = TT, First coin – Head and Second coin – Tail = HT, First coin – Tail and Second coin – Head = TH.
  Thus, the sample space (S) can be represented as {HH, TT, HT, TH}.
• For any random experiment, let S be the sample space. The probability P is a real-valued function whose domain is the power set of S and [0, 1] is the range interval.
• For any event E, P(E) ≥ 0
• P(S) = 1
• If E and F are mutually exclusive events, then P(E ∪ F) = P(E) + P(F).

Events in Probability

As defined above, the set of all possible outcomes is known as the Sample space. All elements of a sample space are known as Sample points. An event is a subset of the S (sample space). An empty set is also known as the Impossible event. The event A′ is known as the Complementary event A.

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All outcomes with equal probability are called equally likely outcomes. Let S be a finite sample space with equally likely outcomes and A be the event. Therefore, the probability of an event A is:
P(A) = n(A)/n(S)
Where n(A) = Number of elements on the set A
n(S) = Total number of outcomes or the number of elements in the sample space S

Let P and Q be any two events, then the following formulas can be derived.

• Event P or Q: The set P ∪ Q
• Event P and Q: The set P ∩ Q
• Event P and not Q: The set P – Q
• P and Q are mutually exclusive if P ∩ Q = φ
• Events P₁, P₂, . . . . . , P_n are exhaustive and mutually exclusive if P₁ ∪ P₂ ∪ . . . . . ∪ P_n = S and E_i ∩ E_j = φ for all i ≠ j.

If P and Q are two events, then P(P or Q) = P(P) + P(Q) – P(P and Q), i.e. P(P ∪ Q) = P(P) + P(Q) – P(P ∩ Q)

• If P and Q are mutually exclusive, then P(P or Q) = P(P) + P(Q)
• If M is an event, then P(not M) = 1 – P(M)
• P(sure event) = 1
• P(impossible event) = 0

Probability Class 11 Practice Questions

Q.1: Two coins (a one rupee coin and a two rupee coin) are tossed once. Find a sample space.

Q.2: Find the sample space associated with the experiment of rolling a pair of dice (one is blue and the other red) once. Also, find the number of elements of this sample space.

Q.3: In each of the following experiments specify appropriate sample space.
(i) A boy has a 1 rupee coin, a 2 rupee coin and a 5 rupee coin in his pocket. He takes two coins out of his pocket, one after the other.
(ii) A person is noting down the number of accidents along a busy highway during a year.

Q.4: A coin is tossed. If it shows head, we draw a ball from a bag consisting of 3 blue and 4 white balls; if it shows tail we throw a die. Describe the sample space of this experiment.

Q.5: Consider the experiment in which a coin is tossed repeatedly until a head comes up. Describe the sample space.

Q.6: Consider the experiment of rolling a die. Let A be the event ‘getting a prime number’, B be the event ‘getting an odd number’. Write the sets representing the events (i) Aor B (ii) A and B (iii) A but not B (iv) ‘not A’.

Q.7: Two dice are thrown and the sum of the numbers which come upon the dice is noted. Let us consider the following events associated with this experiment
A: ‘the sum is even’.
B: ‘the sum is a multiple of 3’.
C: ‘the sum is less than 4’.
D: ‘the sum is greater than 11’.
Which pairs of these events are mutually exclusive?

Q.8: A coin is tossed three times, consider the following events.
A: ‘No head appears’,
B: ‘Exactly one head appears’ and C: ‘At least two heads appear’.
Do they form a set of mutually exclusive and exhaustive events?

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