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Pre-AP Geometry Notes
Name: ______________________
2.1 & 2.4 – Inductive Reasoning vs. Deductive Reasoning
2.1
____________________ _____________________: The process of reasoning that a rule or statement is true
because specific cases are true. When several examples form a pattern and you assume the pattern will
continue, you are applying inductive reasoning.
One may use inductive reasoning to draw a ____________________ from a pattern.
A statement believed to be true based on inductive reasoning is called a ____________________.
Example 1: Complete each conjecture.
a. The product of an even number
b. The number of segments formed
and an odd number is
by n collinear points is
c. The sum of an even number and
an odd number is
___________________.
___________________.
(in terms of n)
___________________.
Example(s):
Example(s):
Example(s):
Example 2: Biology Application: To learn about the migration behavior of California gray whales, biologists observed
whales along two routes. For seven days they counted the numbers of whales seen along each route.
Make a conjecture based on the data.
Numbers of Whales Each Day
Direct Route
1
3
0
2
1
1
0
Shore Route
7
9
5
8
8
6
7

To show that a conjecture is always true, you must ____________________ it.

To show that a conjecture is _______________, you have to find only _____ example in which the
conjecture is not true.

The case, or example, that demonstrates that a conjecture is false, is called a
_________________________. A counterexample can be a drawing, a statement, or a number.
Inductive Reasoning
1. Look for a pattern.
2. Make a conjecture.
3. Prove the conjecture or find a counterexample.
Example 3: Show that each conjecture is false by finding a counterexample.
b. For any three points in a plane, there are three
1
a. For all positive numbers n ,  n .
different lines that contain two of the points.
n
c. For any real number x , x 2  x .
d. Supplementary angles are adjacent.
2.4
____________________ _____________________: the process of using logic to draw conclusions from given
facts, definitions, and properties. Although one counterexample is enough to disprove a conjecture,
to PROVE that a conjecture is true, you must use deductive reasoning.
Example 4: Question: Is each conclusion the result of inductive or deductive reasoning?
a. Mrs. Corlett has never had a dog other than a beagle.
Therefore, in Mrs. Corlett’s life, she will only ever have
beagles.
b. There is a myth that you should not touch a baby bird
that has fallen from its nest because the mother bird will
disown the baby if she detects human scent. However,
biologists have proven that birds cannot detect human
scent. Therefore, the myth cannot be true.
Example 5: Determine if the conjectures are valid or invalid.
a. If a person is in Antarctica, then the person will see a
penguin.
b. Three points that determine a plane also determine a
triangle.
Given: Carlos sees a penguin.
Conjecture: Carlos is in Antarctica.
Given: Lilly creates a plane from points, A, B, and C.
Conjecture: Lilly also created a triangle ABC.
c. If two segments are congruent, then they have the same
length.
d. If you are tardy three or more times, you will have
Saturday School.
Given: AB  XY .
Conjecture: AB  XY
Given: Max has Saturday School.
Conjecture: Max was tardy at least three times.
A _______________ is a logical argument that shows that a conclusion is TRUE.