http://www.ck12.org Chapter 12. Discrete Math
12.7 Basic Probability
Here you will calculate the probability of simple and compound events.
Most are familiar with how flipping a coin or rolling dice works and yet probability remains one of the most
counterintuitive branches of mathematics for many people. The idea that flipping a coin and getting 10 heads in
a row is just as unlikely as getting the following sequence of heads and tails is hard to comprehend.
HHT HT T T HT H
Assume a plane crashes on average once every 100 days (extremely inaccurate). Given a plane crashed today, what
day in the next 100 days is the plane most likely to crash next?
Watch This
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/62255http://www.youtube.com/watch?v=YWt_u5l_jHs James Sousa: Introduction to Probability
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/62259http://www.youtube.com/watch?v=IZAMLgS5x6w James Sousa: Determining Probability
Guidance
Probability is the chance of an event occurring. Simple probability is defined as the number of outcomes you
are looking for (also called successes) divided by the total number of outcomes. The notationP(E)is read “the
probability of eventE”.
P(E) =#possible outcomes#successes
Probabilities can be represented with fractions, decimals, or percents. Since the number of possible outcomes is in
the denominator, the probability is always between zero and one. A probability of 0 means the event will definitely
not happen, while a probability of 1 means the event will definitely happen.
0 ≤P(E)≤ 1
The probability of something not happening is called thecomplementand is found by subtracting the probability
from one.