**The Definition of Probability**

The size of the possible occurence of an event is called the probability of event. The determination of probability’s value of event is based on the number of elements of the event and the number of elements of the sample space.

Suppose in the spinning of a coin, the event shows either the number side or the image side. The number of members is equal to 2. This means that the probability of the numeral side is appear is equal to ½, and the probability of the image side is appear is equal to ½.

Suppose in an experiment every result has the same possibility to occur. If the number of members of the event K=*n*(K) and the number of members of the sample space = *n*(S) then the probability of the event K is

probability formula

Notation for probability is different with permutation. Because probability is able to be a continuous variable, it is written in italic.

Example 2:

a. In throwing of a balance die. Suppose K is the event that the appear of die’s eyes is prime number. Determine the probability of the event K.

b. From a set of a bridge card, one card is taken randomly. Determine the probability of Ace card is taken.

FIGURE ACE CARDS

Answer:

a. The sample spaceof a balance die is:

S={1,2,3,4,5,6}. → the number of members of S:

*n*(S)=6

K=The event that the appear of die’s eyes is prime number

→K={2,3,5}. the number of members of K:

*n*(K)=3.

Then the probability of K is P(K)=3/6= ½

b. A set of bridge card contains of 52 cards →

*n*(S)=52

There are 4 Aces in a set of bridge card, then the number of event’s member of the taken is Ace is

*n*(X)=4.

The probability of the taken card is Ace card =4/52=1/13

**Exercise Competency Test 2**

1. In a throwing experiment of a die in balance, K states the event when the emerging of the die’s eyes is even number. The probability of event K =…

A. ⅙ B. ¼ C. ⅓ D. ½ E. ⅔

Answer: D, Explanation:

2. Suppose we have 10 cards which are numbered 1 to 10. If one card is taken randomly, then the probability of the taken card is card with the number of prime =…

A. ⅘ B. ⅗ C. ½ D. 3/10 E. ⅖

Answer: E, Explanation:

The prime number are {2,3,5,7}. Thus the probability is 4/10 = ⅖.

3. A student holds a set of bridge card which contains 52 cards and he asks his friend to take one card randomly. The probability of the taken card are hearts is …

A. 1/52 B. 1/13 C. 9/52 D. ¼ E. ⅓

Answer: D, Explanation:

A set of bridge card contains 4 types, heart is one of them. The number of each type is equal.

*x*states the emerging of the die’s eyes on top side. The probability (2≤

*x*≤4) is equal to …

A. ⅙ B. ⅓ C. ½ D. ⅔ E. ⅚

Answer: C, Explanation: