Prove that if a, b, and c are positive real numbers with ab = c, then a < Vc or b< Vc.

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question 6。mathematical reasoning. Show steps,probably by p or q is equivalent to not p =》q
1. Prove by contradiction that \(6n + 5\) is odd for all integers \(n\).

2. Prove that for all integers \(n\), if \(3n + 5\) is even then \(n\) is odd. (Hint: prove the contrapositive)

3. Prove that
   \[
   |x + y| \leq |x| + |y|
   \]
   for all real numbers \(x\) and \(y\).

4. Prove that there does not exist a smallest positive real number. (In other words, prove that there does not exist a positive real number \(x\) such that \(x < y\) for all positive real numbers \(y\)).

5. Recall that an irrational number is a real number which is not rational. Prove that if \(x\) is rational and \(y\) is irrational, then \(x + y\) is irrational. You may use the fact that the rational numbers are closed under addition - if \(a\) and \(b\) are rational numbers, then \(a + b\) is rational as well.

6. Prove that if \(a\), \(b\), and \(c\) are positive real numbers with \(ab = c\), then \(a \leq \sqrt{c}\) or \(b \leq \sqrt{c}\).

7. Prove that
   \[
   \sum_{i=1}^n i^3 = \frac{1}{4}n^2(n+1)^2
   \]
   for all positive integers \(n\).

8. Prove that
   \[
   \sum_{i=1}^n i \cdot i! = (n+1)! - 1
   \]
   for all positive integers \(n\).

9. Prove that
   \[
   2^n < n!
   \]
   for all positive integers \(n\) such that \(n \geq 4\).
Transcribed Image Text:1. Prove by contradiction that \(6n + 5\) is odd for all integers \(n\). 2. Prove that for all integers \(n\), if \(3n + 5\) is even then \(n\) is odd. (Hint: prove the contrapositive) 3. Prove that \[ |x + y| \leq |x| + |y| \] for all real numbers \(x\) and \(y\). 4. Prove that there does not exist a smallest positive real number. (In other words, prove that there does not exist a positive real number \(x\) such that \(x < y\) for all positive real numbers \(y\)). 5. Recall that an irrational number is a real number which is not rational. Prove that if \(x\) is rational and \(y\) is irrational, then \(x + y\) is irrational. You may use the fact that the rational numbers are closed under addition - if \(a\) and \(b\) are rational numbers, then \(a + b\) is rational as well. 6. Prove that if \(a\), \(b\), and \(c\) are positive real numbers with \(ab = c\), then \(a \leq \sqrt{c}\) or \(b \leq \sqrt{c}\). 7. Prove that \[ \sum_{i=1}^n i^3 = \frac{1}{4}n^2(n+1)^2 \] for all positive integers \(n\). 8. Prove that \[ \sum_{i=1}^n i \cdot i! = (n+1)! - 1 \] for all positive integers \(n\). 9. Prove that \[ 2^n < n! \] for all positive integers \(n\) such that \(n \geq 4\).
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