diff --git a/chapter_1/1_1/additional_exercises.md b/chapter_1/1_1/additional_exercises.md index e69de29..97ccea4 100644 --- a/chapter_1/1_1/additional_exercises.md +++ b/chapter_1/1_1/additional_exercises.md @@ -0,0 +1,180 @@ +# 1.1.6 Additional Exercises + +1. + +Q: Suppose $P$ and $Q$ are the statements: $P$: Jack passed math. $Q$: Jill +passed math. + +(a) Translate "Jack and Jill both passed math" into symbols. + +(b) Translate "If Jack passed math, then Jill did not" into symbols. + +\(c\) Translate "$P \vee Q$" into English + +(d) Translate "$\neg(P \wedge Q) \to Q$" into English. + +(e) Suppose you know that if Jack passed math, then so did Jill. What can you +conclude if you know that: + +i. Jill passed math? + +ii. Jill did not pass math? + +A: + +(a) Translate "Jack and Jill both passed math" into symbols. + +$$ P \wedge Q $$ + +(b) Translate "If Jack passed math, then Jill did not" into symbols. + +$$ P \to \neg Q $$ + +\(c\) Translate "$P \vee Q$" into English + +"Either Jack passed math or Jill passed math or they both passed math." + +(d) Translate "$\neg(P \wedge Q) \to Q$" into English. + +"If it is true that Jack did not pass math or Jill did not pass math or both did +not pass math, then Jill passed math." + +(e) Suppose you know that if Jack passed math, then so did Jill. What can you +conclude if you know that: + +i. Jill passed math? + +$$ P \to Q $$ + +| $P$ | $Q$ | $P \to Q$ | +| --- | --- | --------- | +| T | T | T | +| T | F | F | +| F | T | T | +| F | F | T | + +While it is true that Jack passed math: + +| $P$ | $Q$ | $P \to Q$ | +| --- | --- | --------- | +| T | T | T | + +It is also possible that Jack did not pass math: + +| $P$ | $Q$ | $P \to Q$ | +| --- | --- | --------- | +| F | T | T | + +Therefore we can say that while it is true that "If Jack passed math, then Jill +passed math." And it is also true that "Jill passed math.", we cannot know if +"Jack passed math" is a true or false statement. + +ii. Jill did not pass math? + +$$ P \to Q $$ + +| $P$ | $Q$ | $P \to Q$ | +| --- | --- | --------- | +| T | F | F | +| F | F | T | + +So given our first assumption, which is that if "Jack passed math, then Jill +passed math", $P \to Q$, is a true statement. We are also told by part ii that +Jill did not pass math. Given this, and given that we know that $P \to Q$ is +true, we can only therefore take the last row: + +| $P$ | $Q$ | $P \to Q$ | +| --- | --- | --------- | +| F | F | T | + +So we know that Jack did not pass math. + +2. + +Q: Translate into symbols. Use $E(x)$ for "$x$ is even" and $O(x)$ for "$x$ is +odd." + +(a) No number is both even and odd. + +(b) One more than any even number is an odd number. + +\(c\) There is a prime number that is even. + +(d) Between any two numbers there is a third number. + +(e) There is no number between a number and one more than that number. + +A: + +(a) No number is both even and odd. + +$$ \forall x \neg (E(x) \wedge O(x)) $$ + +Also: + +$$ \neg \exists x (E(x) \wedge O(x)) $$ + +(b) One more than any even number is an odd number. + +$$ \forall x (E(x) \to O(x + 1)) $$ + +\(c\) There is a prime number that is even. + +Say that $P(x)$ symbolizes "$x$ is prime." then: + +$$ \exists x (P(x) \wedge E(x)) $$ + +(d) Between any two numbers there is a third number. + +This one goes outside the scope of the section a bit, but you can say: + +$$ \forall x \forall z \left(x < z \to \exists y(x < y < z)\right) $$ + +(e) There is no number between a number and one more than that number. + +$$ \forall x \neg \exists y (x < y < x + 1) $$ + +3. + +Q: For each of the statements below, give a domain of discourse for which the +statement is true, and a domain for which the statement is false. + +(a) $\forall x \exists y \left(y^2 = x\right)$. + +(b) $\forall x \forall y \left(x < y \to \exists z(x < z < y)\right)$. + +\(c\) $\exists x \forall y \forall z(y < z \to y \leq x \leq z)$. + +A: + +(a) $\forall x \exists y \left(y^2 = x\right)$. + +The domain of discourse for the statement is true for all real numbers greater +than or equal to $0$. + +$$ \text{DOMAIN WHERE TRUE: } \forall x \exists y \left(y^2 = x\right), x \in \mathbb{R_{\geq 0}} $$ + +The domain of discourse for this statement is false for all real numbers (or +integers) less than $0$ + +$$ \text{DOMAIN WHERE FALSE: } \forall x \exists y \left(y^2 = x\right), x \in \mathbb{R_{<0}} $$ + +(b) $\forall x \forall y \left(x < y \to \exists z(x < z < y)\right)$. + +This is true for the domain of all real numbers: + +$$ x, y, z \in \mathbb{R} $$ + +This is false for the domain of all whole integers, since there is no number +between, say $2$ and $3$ in $\mathbb{Z}$: + +$$ x, y, z \in \mathbb{Z} $$ + +\(c\) $\exists x \forall y \forall z(y < z \to y \leq x \leq z)$. + +This statement is true in any domain with at most one element, and false in any +ordered set with at least two distinct elements. + +Examples: + +True in: $\{0\}$ False in: $\mathbb{Z}, \mathbb{R}$