Chapter 3: Problem 40
The negative of any odd integer is odd.
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Chapter 3: Problem 40
The negative of any odd integer is odd.
These are the key concepts you need to understand to accurately answer the question.
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The square of any integer has the form \(4 k\) or \(4 k+1\) for some integer \(k\).
Observe that $$ \begin{aligned} 7524 &=7 \cdot 1000+5 \cdot 100+2 \cdot 10+4 \\ &=7(999+1)+5(99+1)+2(9+1)+4 \\ &=(7 \cdot 999+7)+(5 \cdot 99+5)+(2 \cdot 9+2)+4 \\ &=(7 \cdot 999+5 \cdot 99+2 \cdot 9)+(7+5+2+4) \\ &=(7 \cdot 111 \cdot 9+5 \cdot 11 \cdot 9+2 \cdot 9)+(7+5+2+4) \\ &=(7 \cdot 111+5 \cdot 11+2) \cdot 9+(7+5+2+4) \\ &=(\text { an integer divisible by } 9) \end{aligned} $$ \(\begin{aligned} 7524 &=7 \cdot 1000+5 \cdot 100+2 \cdot 10+4 \\\ &=7(999+1)+5(99+1)+2(9+1)+4 \\ &=(7.999+7)+(5 \cdot 99+5)+(2 \cdot 9+2)+4 \\\ &=(7 \cdot 999+5 \cdot 99+2 \cdot 9)+(7+5+2+4) \\ &=(7 \cdot 111 \cdot 9+5 \cdot 11 \cdot 9+2 \cdot 9)+(7+5+2+4) \\ &=(7 \cdot 111+5 \cdot 11+2) \cdot 9+(7+5+2+4) \\ &=(\text { an integer divisible by } 9) \\ &+(\text { the sum of the digits of } 7524) \end{aligned}\) Since the sum of the digits of 7524 is divisible by 9,7524 can be written as a sum of two integers each of which is divisible by 9 . It follows from exercise 15 that 7524 is divisible by \(9 .\) Generalize the argument given in this example to any nonnegative integer \(n\). In other words, prove that for any nonnegative integer \(n\), if the sum of the digits of \(n\) is divisible by 9 , then \(n\) is divisible by 9 ,
Given any integer \(n\), if \(n>3\), could \(n, n+2\), and \(n+4\) all be prime? Prove or give a counterexample.
Fill in the blanks in the following proof by contraposition that for all integers \(n\), if \(5 X n^{2}\) then \(5 X n\). Proof (by contraposition): [The contrapositive is: For all integers \(n\), if \(5 \mid n\) then \(5\left\lfloor n^{2}\right.\).] Suppose \(n\) is any integer such that \(\frac{(\mathrm{a})}{-}\) [We must show that (b) ] By definition of divisibility, \(n=\) (c) for some integer \(k\). By substitution, \(n^{2}=\frac{(\mathrm{d})}{-5\left(5 k^{2}\right), \text { But } 5 k^{2} \text { is an integer because it }}\) is a product of integers. Hence \(n^{2}=5 \cdot\) (an integer), and so (e) \([\) as was to be shown \(]\).
For all nonnegative real numbers \(a\) and \(b, \sqrt{a b}=\sqrt{a} \sqrt{b}\). (Note that if \(x\) is a nonnegative real number, then there is a unique nonnegative real number \(y\), denoted \(\sqrt{x}\), such that \(\left.y^{2}=x_{0}\right)\)
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