1. Inductive and Deductive
Reasoning
Topic 1.4.2

2. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
California Standards:
24.1 Students explain the difference between inductive and deductive reasoning and identify and provide examples of each.
24.3 Students use counterexamples to show that an assertion is false and recognize that a single counterexample is sufficient to refute an assertion.
What it means for you:
You’ll identify inductive and deductive reasoning in math problems, and you’ll find how to use counterexamples to disprove a rule.
Key words:
inductive reasoning
deductive reasoning
counterexample

3. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
There are different types of mathematical reasoning.
Two types mentioned in the California math standards are inductive reasoning and deductive reasoning.

4. For example, look at this sequence of square numbers:
Topic
1.4.2
Inductive and Deductive Reasoning
Inductive Reasoning Means Finding a General Rule
Inductive reasoning means finding a general rule by considering a few specific cases.
For example, look at this sequence of square numbers:
1, 4, 9, 16, 25, 36...
If you look at the differences between successive terms, you find this:
The difference between the 1st and 2nd terms is 4 – 1 = 3.
The difference between the 2nd and 3rd terms is 9 – 4 = 5.
The difference between the 3rd and 4th terms is 16 – 9 = 7.
The difference between the 4th and 5th terms is 25 – 16 = 9.

5. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
If you look at these differences:
3, 5, 7, 9
… there’s a pattern — each difference is an odd number, and each one is 2 greater than the previous difference.
Using inductive reasoning, you might conclude that:
The difference between successive square numbers is always odd, and each difference is 2 greater than the previous one.
Watch out though — this doesn’t actually prove the rule.
This rule does look believable, but to prove it you’d have to use algebra.

6. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Guided Practice
Use inductive reasoning to work out an expression for the nth term (xn) of these sequences. For example, the formula for the nth term of the sequence 1, 2, 3, 4,... is xn = n.
1. 2, 3, 4, 5, , 12, 13, 14,...
3. 2, 4, 6, 8, –1, –2, –3, –4,...
In Exercises 5-6, predict the next number in each pattern.
5. 1 = 12, = 22, = 32, = ?
6. 1, 1, 2, 3, 5, 8, 13, …
xn = n + 1
xn = n + 10
xn = 2n
xn = –n 42
= 21
Solution follows…

7. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
One Counterexample Proves That a Rule Doesn’t Work
If you are testing a rule, you only need to find one counterexample (an example that does not work) to prove that the rule is not true.
Once you have found one counterexample, you don’t need to look for any more — one is enough.

8. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Example 1
Decide whether the following statement is always true: “2n + 1 is always a prime number, if n is a natural number.”
Solution
At first, the rule looks believable.
If n = 1: 2n + 1 = = = 3.
This is a prime number, so the rule holds for n = 1.
If n = 2: 2n + 1 = = = 5.
This is a prime number, so the rule holds for n = 2.
Solution continues…
Solution follows…

9. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Example 1
Decide whether the following statement is always true: “2n + 1 is always a prime number, if n is a natural number.”
Solution (continued)
If n = 3: 2n + 1 = = = 9.
This is not a prime number, so the rule doesn’t hold for n = 3.
So a counterexample is n = 3, and this proves that the rule is not always true.
Once you’ve found one counterexample, you don’t need to find any more.

10. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Guided Practice
Give a counterexample to disprove each of the following statements.
7. All odd numbers are of the form 4n + 1, where n is a natural number.
8. If a number is divisible by both 6 and 3, then it is divisible by 12.
9. |x + 4| ³ 4
10. The difference between any two square numbers is always odd.
11. The difference between any two prime numbers is always even.
For example, 3.
For example, 6.
For example, x = –4.
For example, 25 – 9 = 16.
For example, 3 – 2 = 1.
Solution follows…

11. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Guided Practice
Give a counterexample to disprove each of the following statements.
12. The difference between two consecutive cube numbers is always prime.
13. All quadrilaterals are squares.
14. All angles are right angles.
15. All prime numbers are odd.
16. A number is always greater than its multiplicative inverse.
For example, 63 – 53 = 216 – 125 = 91 (= 7 × 13).
For example, rectangles, trapezoids, kites.
For example, any obtuse or acute angle.
For example, 2.
For example, ¼ < 4.
Solution follows…

12. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Deductive Reasoning Means Applying a General Rule
Deductive reasoning is almost the opposite of inductive reasoning.
In deductive reasoning, you use a general rule to find out a specific fact.

13. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Example 2
A number is a multiple of 3 if the sum of its digits is a multiple of 3. Use this information to decide whether 96 is a multiple of 3.
Solution
The sum of the digits is = 15, which is divisible by 3.
The statement says that a number is a multiple of 3 if the sum of its digits is a multiple of 3.
Using deductive reasoning, that means that you can say that 96 is divisible by 3.
Solution follows…

14. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Guided Practice
Use deductive reasoning to work out the 10th term of these sequences:
17. xn = n xn = 6n
19. xn = 2n – xn = 3n – 1
21. xn = 20n xn = n(n + 1)
Use deductive reasoning to reach a conclusion:
23. Ivy is older than Peter. Stephen is younger than Peter.
24. Lily lives in Maryland. Maryland is in the United States. 10 60 19 29
201
110
Ivy is older than Stephen.
Lily lives in the United States.
Solution follows…

15. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Independent Practice
Use inductive reasoning in Exercises 1–2.
1. Give the next three numbers of the sequence: 25, 29, 34, 40, …
2. Write an expression for the nth term (xn) of the sequence: , 72, 216, 648, …
47, 55, 64
xn = 3xn–1
or, xn = 24 • 3xn–1
Solution follows…

16. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Independent Practice
3. Audrey needs $650 to buy a digital camera. Her savings account shows the following balances:
Date
Balance
Jan 1
Feb 1
Mar 1
Apr 1
$100.00
$150.00
$250.00
$400.00
If the pattern continues, at the start of which month will she be able to buy the digital camera? Use inductive reasoning.
June
Solution follows…

17. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Independent Practice
Use deductive reasoning in exercises 4-5.
4. Find the first five terms of the sequence xn = 3n.
5. Find the first five terms of the sequence xn = n(n – 1).
3, 6, 9, 12, 15
0, 2, 6, 12, 20
Solution follows…

18. Inductive and Deductive Reasoning
Topic
1.4.2
Inductive and Deductive Reasoning
Round Up
Inductive reasoning means that you can make a general rule without having to check every single value — so it saves you a lot of work.