8. Suppose a is an integer. If 5| 2a, then 5|a.

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Question 8 please write out 

proof, assume , therefor etc 

**Exercises for Chapter 4**

*Use the method of direct proof to prove the following statements:*

1. If \( x \) is an even integer, then \( x^2 \) is even.

2. If \( x \) is an odd integer, then \( x^3 \) is odd.

3. If \( a \) is an odd integer, then \( a^2 + 3a + 5 \) is odd.

4. Suppose \( x, y \in \mathbb{Z} \). If \( x \) and \( y \) are odd, then \( xy \) is odd.

5. Suppose \( x, y \in \mathbb{Z} \). If \( x \) is even, then \( xy \) is even.

6. Suppose \( a, b, c \in \mathbb{Z} \). If \( a \mid b \) and \( a \mid c \), then \( a \mid (b + c) \).

7. Suppose \( a, b \in \mathbb{Z} \). If \( a \mid b \), then \( a^2 \mid b^2 \).

8. Suppose \( a \) is an integer. If \( 5 \mid 2a \), then \( 5 \mid a \).

9. Suppose \( a \) is an integer. If \( 7 \mid 4a \), then \( 7 \mid a \).

10. Suppose \( a \) and \( b \) are integers. If \( a \mid b \), then \( a \mid (3b^3) \).

11. Suppose \( a, b, c, d \in \mathbb{Z} \). If \( a \mid b \) and \( c \mid d \), then \( ac \mid bd \).

12. If \( x \in \mathbb{R} \) and \( 0 < x < 4 \), then \(\frac{4}{x(4-x)} \geq 1\).

13. Suppose \( x, y \in \mathbb{R} \). If \( x^2 + 5y = y^2 + 5x \), then \( x = y \) or \( x + y = 5 \).

14. If \( n \in \mathbb
Transcribed Image Text:**Exercises for Chapter 4** *Use the method of direct proof to prove the following statements:* 1. If \( x \) is an even integer, then \( x^2 \) is even. 2. If \( x \) is an odd integer, then \( x^3 \) is odd. 3. If \( a \) is an odd integer, then \( a^2 + 3a + 5 \) is odd. 4. Suppose \( x, y \in \mathbb{Z} \). If \( x \) and \( y \) are odd, then \( xy \) is odd. 5. Suppose \( x, y \in \mathbb{Z} \). If \( x \) is even, then \( xy \) is even. 6. Suppose \( a, b, c \in \mathbb{Z} \). If \( a \mid b \) and \( a \mid c \), then \( a \mid (b + c) \). 7. Suppose \( a, b \in \mathbb{Z} \). If \( a \mid b \), then \( a^2 \mid b^2 \). 8. Suppose \( a \) is an integer. If \( 5 \mid 2a \), then \( 5 \mid a \). 9. Suppose \( a \) is an integer. If \( 7 \mid 4a \), then \( 7 \mid a \). 10. Suppose \( a \) and \( b \) are integers. If \( a \mid b \), then \( a \mid (3b^3) \). 11. Suppose \( a, b, c, d \in \mathbb{Z} \). If \( a \mid b \) and \( c \mid d \), then \( ac \mid bd \). 12. If \( x \in \mathbb{R} \) and \( 0 < x < 4 \), then \(\frac{4}{x(4-x)} \geq 1\). 13. Suppose \( x, y \in \mathbb{R} \). If \( x^2 + 5y = y^2 + 5x \), then \( x = y \) or \( x + y = 5 \). 14. If \( n \in \mathbb
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