Probability Class 9 Notes CBSE Maths Chapter 15 (Free PDF Download)

Probability Formula

Suppose the total number of trials is ‘n’ then the probability of an event (D) occurring will be: P (D) = number of trials in which the event occurred/ total number of trials.

Summary of Notes of Class 9 Revision Notes Chapter 15- Probability

Here, you will get the requisite definition, concepts, and problems on the concepts of probability. Below are some of the important topics you will need to cover from class 9 maths notes of probability.

Uses And Application of Probability

Probability is widely used in various fields, such as, Physical Sciences, Medical Sciences, Biological Sciences, Commerce, Weather Forecasting, Mathematics, etc.

Standard Terms Related To Probability

1. Randomness

If we are conducting an experiment and we are not aware of the next result of the experiment to happen then it is known as the Random Experiment.

2. Trial

A trial is that action whose result is not just one but there may be one or more results. For example, pull the card of spade 7 from the deck of cards or the outcome of a throw of a dice, etc.

3. Independent Trial

A trial is said to be independent if it does not affect the result of any other random trial. Like tossing a coin or throwing a dice are independent trials as they are not bound to impact each other.

4. Event

While performing an experiment, an event will be the gathering of some results of that experiment. For example, when we throw a dice, then the possibility of having an outcome of an odd number will be three, i.e., 1, 3, 5. So the event would comprise of three outcomes.

5. Impossible Events

While performing a test if an event is not possible to take place then its probability will be zero (0). This is called an Impossible Event. For example, you cannot throw dice with the outcome of number eight on it. Thus, the probability of getting an 8 on dice is 0%.

6. Sure or Certain Event

While carrying out a test if there is a certainty of an event to take place, then it is said to be a certain probability. Here the probability is 1. For example, it is sure to draw a red ball from a bag that contains red balls only.

This proves that the probability of an event could be between 1 and 0. Thus, 0 ≤ P (E) ≤ 1.

7. Elementary Event

If there is only one possible outcome of an event to take place, then it is what we call an Elementary Event. For example, if we add all the elementary events of an experiment then their sum will be 1.

The general form is as given:

P (H) + P (T) = 1

P (H) + P= 1 (where H- ‘not H’). 

P (H) – 1 = P

P (H) and P are called complementary events.

Solved Examples Included In Revision Notes Class 9 Maths Chapter 15

You will find the following types of problems on probability in your Maths Class 9 Probability Notes.

Example 1

If a coin is tossed 200 times out of which 70 times we get heads and 130 times we get tails. Then,

1. Find the probability of getting a heads.

2. Find the probability of getting a tail.

3. Check if the sum of the two probabilities is equivalent to 1 or not.

Solution 1

a. Let the probability of getting a heads on the coin be P (H)

Thus, applying the formula:

P (H) = number of trials in which the head comes/ total number of trials

= 70/200

=0.35

b. Let the probability of getting a heads on the coin be P (T)

Thus, applying the formula:

P (T) = number of trials in which the tail comes/ total number of trials

= 130/200

=0.65

c. The sum of 2 probability will be = P(H) + P(T)

= 70/200 + 130/200 = 200/200 = 1

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