1. Copyright © 2016, 2013, and 2010, Pearson Education, Inc.4
Chapter
Number Theory
Copyright © 2016, 2013, and 2010, Pearson Education, Inc.

2. 4-1 Divisibility Students will be able to understand and explain
• Divisibility, factors, and multiples.
• Divisibility tests for 2, 3, 4, 5, 6, 8, 9, 10, and 11.
• That the set of factors is finite, and the set of multiples is infinite for any given natural number.

3. Divisibility
An integer is even if it has a remainder of 0 when divided by 2; it is odd otherwise.
We say that “3 divides 18”, written 3 | 18, because the remainder is 0 when 18 is divided by 3.
Likewise, “b divides a” can be written b | a.
We say that “3 does not divide 25”, written , because the remainder is not 0 when 25 is divided by 3.

4. Divisibility
In general, if a is a nonnegative integer and b is a positive integer, we say that a is divisible by b, or b divides a if and only if the remainder is 0 when a is divided by b.

5. Definition
For any whole numbers a and b, where b ≠ 0, b divides a, if, and only if, there is a unique while number q such that a = bq.
If b | a, then b is a factor or a divisor of a, and a is a multiple of b.

6. Example Classify each of the following as true or false. a. 3 | 12
b. 0 | 2
False
c. 0 is even.
True d.
True
e. For all whole numbers a, 1 | a.
True
f. For all non-zero whole numbers a, a2 | a5.
True
g. 3 | 6n for all integers n.
True

7. Example (continued) h.
True
g. 0 | 0
False

8. Properties of Division
For any whole numbers a and d, if d | a, and n is any whole number, then d | na.
In other words, if d is a factor of a, then d is a factor of any multiple of a.

9. Properties of Division
For any integers a, b, and d, d ≠ 0:
a. If d | a, and d | b, then d | (a + b).
b. If d | a, and , then
c. If d | a, d | b, and a > b, then d | (a − b).
d. If d | a, , and a > b then
e. If d a, d | b, and a > b, then

10. Example
Classify each of the following as true or false, where x, y, and z are whole numbers.
a. If 3 | x and 3 | y, then 3 | xy.
True
b. If 3 | (x + y), then 3 | x and 3 | y.
False c.
False

11. Divisibility Rules
Sometimes it is useful to know if one number is divisible by another just by looking at it.
For example, to check the divisibility of 1734 by 17, we note that 1734 = We know that 17 | 1700 because 17 | 17 and 17 divides any multiple of 17.
Furthermore, 17 | 34; therefore, we conclude that 17 | 1734.

12. Divisibility Rules
Another method to check for divisibility is to use the integer division button on a calculator.
INT ÷
Press the following sequence of buttons:
INT ÷ 1 7 3 4 =
to obtain the display Q R

13. Divisibility Tests
A whole number is divisible by 2 if and only if its units digit is divisible by 2.
A whole number is divisible by 5 if and only if its units digit is divisible by 5, that is if and only if the units digit is 0 or 5.
A whole number is divisible by 10 if and only if its units digit is divisible by 10, that is if and only if the units digit is 0.

14. Divisibility Tests
A whole number is divisible by 4 if and only if the last two digits of the number represent a number divisible by 4.
A whole number is divisible by 8 if and only if the last three digits of the whole number represent a number divisible by 8.

15. Example Determine whether 97,128 is divisible by 2, 4, and 8.
2 | 97,128 because 2 | 8.
4 | 97,128 because 4 | 28.
8 | 97,128 because 8 | 128.

16. Example Determine whether 83,026 is divisible by 2, 4, and 8.
2 | 83,026 because 2 | 6.

17. Divisibility Tests

18. Divisibility Tests
A whole number is divisible by 3 if and only if the sum of its digits is divisible by 3.
A whole number is divisible by 9 if and only if the sum of the digits of the whole number is divisible by 9.

19. Example Determine whether 1002 is divisible by 3 and 9.
Because = 3 and 3 | 3, 3 | 1002.
Because

20. Example Determine whether 14,238 is divisible by 3 and 9.
Because = 18 and 3 | 18, 3 | 14,238.
Because 9 | 18, 9 | 14,238.

21. Example
The store manager has an invoice for 72 calculators. The first and last digits on the receipt are illegible. The manager can read
$■67.9■
What are the missing digits, and what is the cost of each calculator?

22. Example (continued)
Let the missing digits be represented by x and y, so that the number is x67.9y dollars, or x679y cents.
Because 72 calculators were sold, the amount must be divisible by 72.
Because 72 = 8 · 9, the amount is divisible by both 8 and 9.

23. Example (continued)
For the number on the invoice to be divisible by 8, the three-digit number 79y must be divisible by 8.
Only 792 is divisible by 8, so y = 2, and the last digit on the invoice is 2.

24. Example (continued)
Because the number on the invoice must be divisible by 9, we know that 9 must divide x , or x + 24.
Since 3 is the only single digit that will make x + 24 divisible by 9, x = 3.
The number on the invoice must be $
The calculators cost $ ÷ 72 = $5.11, each.

25. Divisibility Tests
A whole number is divisible by 11 if and only if the sum of the digits in the places that are even powers of 10 minus the sum of the digits in the places that are odd powers of 10 is divisible by 11.
An integer is divisible by 6 if and only if the whole number is divisible by both 2 and 3.

26. Example
The number 57,729,364,583 has too many digits for most calculator displays. Determine whether it is divisible by each of the following:
a. 2 No
b. 3 No
c. 5 No
d. 6 No
e. 8 No
f. 9 No
g. 10 No
h. 11
Yes