# 12.3 An Introduction to Probability

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12.3 An Introduction to Probability
What is probability? What is the difference between theoretical probability and experimental probability? What is geometric probability?

The probability of an event is a number between
THEORETICAL AND EXPERIMENTAL PROBABILITY The probability of an event is a number between 0 and 1 that indicates the likelihood the event will occur. There are two types of probability: theoretical and experimental.

THEORETICAL AND EXPERIMENTAL PROBABILITY
THE THEORETICAL PROBABILITY OF AN EVENT The theoretical probability of an event is often simply called the probability of the event. When all outcomes are equally likely, the theoretical probability that an event A will occur is: number of outcomes in A P (A) = 4 9 P (A) = total number of outcomes all possible outcomes outcomes in event A You can express a probability as a fraction, a decimal, or a percent. For example: , 0.5, or 50%. 1 2

1 6 Find the probability of rolling a 4. SOLUTION
Finding Probabilities of Events You roll a six-sided die whose sides are numbered from 1 through 6. Find the probability of rolling a 4. SOLUTION Only one outcome corresponds to rolling a 4. number of ways to roll a 4 1 6 = P (rolling a 4) = number of ways to roll the die

3 1 6 2 Find the probability of rolling an odd number. SOLUTION
Finding Probabilities of Events You roll a six-sided die whose sides are numbered from 1 through 6. Find the probability of rolling an odd number. SOLUTION Three outcomes correspond to rolling an odd number: rolling a 1, 3, or a 5. number of ways to roll an odd number 3 6 1 2 = = P (rolling odd number) = number of ways to roll the die

Find the probability of rolling a number less than 7.
Finding Probabilities of Events You roll a six-sided die whose sides are numbered from 1 through 6. Find the probability of rolling a number less than 7. SOLUTION All six outcomes correspond to rolling a number less than 7. number of ways to roll less than 7 6 = = 1 P (rolling less than 7 ) = number of ways to roll the die

SOLUTION What is the probability that the songs are played
Probabilities Involving Permutations or Combinations You put a CD that has 8 songs in your CD player. You set the player to play the songs at random. The player plays all 8 songs without repeating any song. What is the probability that the songs are played in the same order they are listed on the CD? Help SOLUTION There are 8! different permutations of the 8 songs. Of these, only 1 is the order in which the songs are listed on the CD. So, the probability is: 1 8! 40, 320 P(playing 8 in order) = = 

Probabilities Involving Permutations or Combinations
You put a CD that has 8 songs in your CD player. You set the player to play the songs at random. The player plays all 8 songs without repeating any song. You have 4 favorite songs on the CD. What is the probability that 2 of your favorite songs are played first, in any order? Help SOLUTION There are 8C2 different combinations of 2 songs. Of these, 4C2 contain 2 of your favorite songs. So, the probability is: P(playing 2 favorites first) = = =  4 C 2 8 C 2 6 28 3 14

Probabilities Involving Permutations or Combinations
Sometimes it is not possible or convenient to find the theoretical probability of an event. In such cases you may be able to calculate an experimental probability by performing an experiment, conducting a survey, or looking at the history of the event.

In 1998 a survey asked Internet users for their ages. The
Finding Experimental Probabilities In 1998 a survey asked Internet users for their ages. The results are shown in the bar graph.

Of the people surveyed, 16 36 are at most 20 years old.
Finding Experimental Probabilities Find the experimental probability that a randomly selected Internet user is at most 20 years old. 1636 6617 3693 491 6 SOLUTION The number of people surveyed was = 12,443. Of the people surveyed, are at most 20 years old. So, the probability is: P(user is at most 20) =  0.131 1636 12,443

Of the people surveyed, 3693 + 491 + 6 = 4190 are
Finding Experimental Probabilities Find the experimental probability that a randomly selected Internet user is at least 41 years old. Given that 12,443 people were surveyed. SOLUTION Of the people surveyed, = are at least 41 years old. So, the probability is: P(user is at least 41) =  0.337 4190 12,443

GEOMETRIC PROBABILITY
Some probabilities are found by calculating a ratio of two lengths, areas, or volumes. Such probabilities are called geometric probabilities.

You throw a dart at the board
Using Area to Find Probability You throw a dart at the board shown. Your dart is equally likely to hit any point inside the square board. Are you more likely to get 10 points or 0 points?

You are more likely to get 0 points.
Using Area to Find Probability Are you more likely to get 10 points or 0 points? SOLUTION area of smallest circle P (10 points) = area of entire board  • 32 182 = = =  324 9  36 area outside largest circle P (0 points) = area of entire board 182 – ( • 9 2 ) 182 = = =  0.215 324 324 – 81 4 4 –  You are more likely to get 0 points.

What is probability? A number between 0 and 1 of the likelihood that an event will occur. What is the difference between theoretical probability and experimental probability? When all outcomes are equally likely (theoretical) compared to the actual information collected by performing an experiment, conducting a survey or looking at the history of an event (experimental). What is geometric probability? Probabilities using the ratio of two lengths, areas or volumes.

Assignment: P ,

COMBINATIONS OF n OBJECTS TAKEN r AT A TIME
Back Back COMBINATIONS OF n OBJECTS TAKEN r AT A TIME The number of combinations of r objects taken from a group of n distinct objects is denoted by nCr and is given by: nCr = n ! (n – r )! • r ! For instance, the number of combinations of 2 objects taken from a group of 5 objects is 5C 2 = 5! 3! • 2!

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