Chapter 3: Problem 54
The difference of the squares of any two consecutive integers is odd.
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Chapter 3: Problem 54
The difference of the squares of any two consecutive integers is odd.
These are the key concepts you need to understand to accurately answer the question.
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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 ,
Assume that \(m\) and \(n\) are particular integers. \(\begin{array}{ll}\text { a. Is } 6 m+8 n \text { even? } & \text { b. Is } 10 m n+7 \text { odd? }\end{array}\) c. If \(m>n>0\), is \(m^{2}-n^{2}\) composite?
If \(0=\) Sunday, \(1=\) Monday, \(2=\) Tuesday, \(\ldots, 6=\) Saturday, then January 1 of year \(n\) occurs on the day of the week given by the following formula: \(\left(n+\left\lfloor\frac{n-1}{4}\right\rfloor-\left\lfloor\frac{n-1}{100}\right\rfloor+\left\lfloor\frac{n-1}{400}\right\rfloor\right) \bmod 7\). a. Use this formula to find January 1 of \(\begin{array}{lll}\text { i. } 2050 & \text { ii. } 2100 & \text { iii. the year of your birth. }\end{array}\)
Prove that for all real numbers \(c\), if \(c\) is a root of a polynomial with rational coefficients, then \(c\) is a root of a polynomial with integer coefficients.
The difference of any two odd integers is even.
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