Geometry Geometric Probability
Jan 03, 2016
Geometry
Geometric Probability
April 20, 2023
Goals
Know what probability is. Use areas of geometric figures to
determine probabilities.
April 20, 2023
Probability
A number from 0 to 1 that represents the chance that an event will occur.
P(E) means “the probability of event E occuring”.
P(E) = 0 means it’s impossible. P(E) = 1 means it’s certain. P(E) may be given as a fraction,
decimal, or percent.
April 20, 2023
Probability
Number of Successful OutcomesP(E)=
Total number of Outcomes
Example
A ball is drawn at random from the box. What is the probability it is red?
P(red) = ??29
April 20, 2023
Probability
Number of Successful OutcomesP(E)=
Total number of Outcomes
A ball is drawn at random from the box. What is the probability it is green or black?
P(green or black) = ??39
April 20, 2023
Probability
Number of Successful OutcomesP(E)=
Total number of Outcomes
A ball is drawn at random from the box. What is the probability it is green or black?
P(green or black) = 13
April 20, 2023
Geometric Probability
Based on lengths of segments and areas of figures.
Random:Without plan or order. There is no bias.
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Probability and Length
Let AB be a segment that contains the segment CD. If a point K on AB is chosen at random, then the probability that it is on CD is
Length of CDP(K is on CD)
Length of AB
April 20, 2023
Example 1
Find the probability that a point chosen at random on RS is on JK.
JK = 3
RS = 9
Probability = 1/3
1 2 3 4 5 6 7 8 9 10 11 12
R SJ K
April 20, 2023
Your Turn
Find the probability that a point chosen at random on AZ is on the indicated segment.
15
25
12
AB
AC
BD
1 2 3 4 5 6 7 8 9 10 11 12
A ZB C D E
910
110
45
AE
EZ
BZ
April 20, 2023
Probability and Area
Let J be a region that contains region M. If a point K in J is chosen at random, then the probability that it is in region M is
Area of MP(K is in M)=
Area of J
MJ
K
April 20, 2023
Example 2
Find the probability that a randomly chosen point in the figure lies in the shaded region.
8
8
April 20, 2023
Example 2 Solution
8
8
Area of Square = 82 = 64
Area of Triangle
A=(8)(8)/2 = 32
Area of shaded region
64 – 32 = 32
Probability:
32/64 = 1/2
8
April 20, 2023
Example 3Find the probability that a randomly chosen point in the figure lies in the shaded region.
5
April 20, 2023
Example 3 Solution
5510
Area of larger circle
A = (102) = 100
Area of one smaller circle
A = (52) = 25
Area of two smaller circles
A = 50
Shaded Area
A = 100 - 50 = 50
Probability50 1
100 2
April 20, 2023
Your TurnA regular hexagon is inscribed in a circle. Find the probability that a randomly chosen point in the circle lies in the shaded region.
6
April 20, 2023
Solution
3 3
121
3 3 362
54 3 93.53
A ap
6 ?6
?3?3 3
Find the area of the hexagon:
April 20, 2023
Solution
3 36 6
33 3
Find the area of the circle:
A = r2
A=36 113.1
Shaded Area
Circle Area – Hexagon Area
113.1 – 93.63 =19.57
113.1
19.57
93.53
April 20, 2023
Solution
3 36 6
33 3
Probability:
Shaded Area ÷ Total Area
19.57/113.1 = 0.173
17.3%113.1
19.57
April 20, 2023
Example 4
If 20 darts are randomly thrown at the target, how many would be expected to hit the red zone?
10
April 20, 2023
Example 4 Solution
10
Radius of small circles:
5
Area of one small circle:
25
Area of 5 small circles:
125
April 20, 2023
Example 4 Solution continued
10
Radius of large circle:
15
Area of large circle:
(152) = 225
Red Area:
(Large circle – 5 circles)
225 125 = 100
10
5
April 20, 2023
Example 4 Solution continued
10
Red Area:100
Total Area: 225
Probability:
100 40.444...
225 9
This is the probability for each dart.
April 20, 2023
Example 4 Solution continued
10
Probability:
100 40.444...
225 9
For 20 darts, 44.44% would likely hit the red area.
20 44.44% 8.89, or about 9 darts.
April 20, 2023
Your Turn
500 points are randomly selected in the figure. How many would likely be in the green area?
5 3
April 20, 2023
Solution500 points are randomly selected in the figure. How many would likely be in the green area?
5 3
10
Area of Hexagon:
A = ½ ap
A = ½ (53)(60)
A = 259.81
Area of Circle:
A = r2
A = (53)2
A= 235.62
60
30
5
10
April 20, 2023
Solution500 points are randomly selected in the figure. How many would likely be in the green area?
5 3
Area of Hexagon:
A = 259.81
Area of Circle:
A= 235.62
Green Area:
259.81 – 235.62
24.19
April 20, 2023
Solution500 points are randomly selected in the figure. How many would likely be in the green area?
5 3
Area of Hexagon:
A = 259.81
Green Area:
24.19
Probability:
24.19/259.81 =
0.093 or 9.3%
April 20, 2023
Solution500 points are randomly selected in the figure. How many would likely be in the green area?
5 3
Probability:
0.093 or 9.3%
For 500 points:
500 .093 = 46.5
47 points should be in the green
area.
April 20, 2023
Summary
Geometric probabilities are a ratio of the length of two segments or a ratio of two areas.
Probabilities must be between 0 and 1 and can be given as a fraction, percent, or decimal.
Remember the ratio compares the successful area with the total area.
April 20, 2023
Practice Problems