**1.** The science which measures the degree of uncertainty is called probability.

**2.** There are two types of approaches to the study of probability. These are experimental or empirical approach and theoretical approach.

**3.** In the experimental approach to probability, we find the probability of the occurrence of an event by actually performing the experiment a number of times and record the happening of an event.

**4.** In the theoretical approach to probability, we predict the results without actually performing the experiment.

**5.** The observations of an experiment are called its outcomes.

**6. **An experiment in which all possible outcomes are known and the exact outcome cannot be predicted in advance, is called a **random experiment**.

**7.** The word unbiased means each outcome is equally likely to occur. For example, an unbiased die indicates that each of the outcomes 1, 2, 3, 4, 5 or 6 has equal chances to occur. Throughout this chapter, we shall assume that all the experiments have equally likely outcomes.

**8.** The theoretical probability of an event E, written as P(E) is defined as

**9.** An event having only one outcome of the experiment is called an elementary event.

**10.** The sum of the probabilities of all the elementary events of an experiment is 1.

In general for any event E

or

or

Here the event , representing not E, is called the compliment of the event E.

**11.** The probability of the event which is impossible to occur is 0. Such an event is called an impossible event.

**12. **The probability of an event which is sure (or certain) to occur is 1. Such an event is called a sure or a certain event.

**13.** For an event E, we have 0 < P(E) < 1.

**14.** A die is a well balanced cube with its six faces marked with numbers or dots 1 to 6. When we throw a die we are interested in the number that occurs on the top face.

**15. **The pack or deck of playing cards consists of 52 cards, 26 of red colour and 26 of black colour. There are four suits each of 13 cards namely hearts , spades , diamonds and clubs. Each suit contains ace, king, queen, jack or knave, 10, 9, 8, 7, 6, 5, 4, 3, 2.

There are 4 aces, 4 kings, 4 queens, 4 jacks, 4 tens, and so on in a pack. Kings, queens, and jacks are called face cards.

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