FAVOURABLE ELEMENTARY EVENTS An elementary event is said to be favourable to a compound event A, if it satisfies the definition of the compound event A.
In other words, an elementary event E is favourable to a compound event A, if we say that the event A occurs when E is an outcome of a trial.
Consider the random experiment of throwing a pair of dice and the compound event A defined by “Getting 8 as the sum.” We observe that the event A occurs if we get any one of the following elementary events as outcome:
(2, 6), (6, 2), (3, 5), (5, 3), (4,4)
So, there are 5 elementary events favourable to event A.
11 two coins are tossed simultaneously and A is an event associated to it defined as “etting exactly one head”. We say that the event A occurs if we get either HT or TH as an outcome. So, there are two elementary events favourable to the event A.
NEGATION OF AN EVENT Corresponding to every event A associated with a random experiment we define an event “not A” which occurs when and only when A does not occur. The event “not A” is called the negation of event A and is denoted by A.
Clearly, event A occurs if and only if A does not occur.