Let S be the statement "The square of any rational number is rational." A formal version of S is "For every rational number r, r² is rational." Fill in the blanks in the proof for S. Proof: Suppose that r is (a) By definition of rational, r = a/b for some (b) with b 0. By = substitution, p² (c) =a²/b². = Since a and b are both integers, so are the prod- ucts a² and_ (d). Also b² #0 by the (e) Hence r² is a ratio of two integers with a non- zero denominator, and so rational. by definition of

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**Exercise 11**  
Prove that every integer is a rational number.

**Exercise 12**  
Let \( S \) be the statement "The square of any rational number is rational." A formal version of \( S \) is "For every rational number \( r \), \( r^2 \) is rational." Fill in the blanks in the proof for \( S \).

**Proof:**  
Suppose that \( r \) is \((a)\). By definition of rational, \( r = a/b \) for some \((b)\) with \( b \neq 0 \). By substitution, 

\[ r^2 = \frac{(c)}{} = \frac{a^2}{b^2}. \]

Since \( a \) and \( b \) are both integers, so are the products \( a^2 \) and \((d)\). Also \( b^2 \neq 0 \) by the \((e)\). Hence \( r^2 \) is a ratio of two integers with a non-zero denominator, and so \((f)\) by definition of rational.

**Exercise 13**  
Consider the following statement: The negative of any rational number is rational.  
a. Write the statement formally using a quantifier and a variable.  
b. Determine whether the statement is true or false and justify your answer.

**Exercise 14**  
Consider the statement: The cube of any rational number is a rational number.  
a. Write the statement formally using a quantifier and a variable.
Transcribed Image Text:**Exercise 11** Prove that every integer is a rational number. **Exercise 12** Let \( S \) be the statement "The square of any rational number is rational." A formal version of \( S \) is "For every rational number \( r \), \( r^2 \) is rational." Fill in the blanks in the proof for \( S \). **Proof:** Suppose that \( r \) is \((a)\). By definition of rational, \( r = a/b \) for some \((b)\) with \( b \neq 0 \). By substitution, \[ r^2 = \frac{(c)}{} = \frac{a^2}{b^2}. \] Since \( a \) and \( b \) are both integers, so are the products \( a^2 \) and \((d)\). Also \( b^2 \neq 0 \) by the \((e)\). Hence \( r^2 \) is a ratio of two integers with a non-zero denominator, and so \((f)\) by definition of rational. **Exercise 13** Consider the following statement: The negative of any rational number is rational. a. Write the statement formally using a quantifier and a variable. b. Determine whether the statement is true or false and justify your answer. **Exercise 14** Consider the statement: The cube of any rational number is a rational number. a. Write the statement formally using a quantifier and a variable.
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