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October 15, 2013 01:10
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% LaTeX document template for CIS 275 homework assignments. | |
% Dan McArdle | |
% 10-14-2013 | |
\documentclass[12pt]{article} | |
\usepackage[margin=1in]{geometry} | |
\usepackage{amsmath,amsthm,amssymb} | |
\begin{document} | |
% Boilerplate for creating a header | |
\title{Weekly Homework \#1} | |
\author{Dan McArdle\\ | |
CIS 275 -- Discrete Math} | |
\date{Oct 14, 2013} | |
\maketitle % without this line, the header would not be generated | |
% We're going to be writing a lot of backslashes, so let's avoid writing \backslash a lot by creating a macro. | |
\newcommand{\bs}{\backslash} | |
\section*{Problem 5} | |
\paragraph{Claim:} Let $A$, $B$, $C$ be sets. Then, $(A\bs B)\bs C \subseteq (A\bs C)\bs (B\bs C)$. | |
\paragraph{Proof: (direct)} | |
It is sufficient to show the following two subclaims. | |
\begin{enumerate} | |
\item $(A\bs B)\bs C \subseteq (A\bs C)\bs (B\bs C)$ | |
\item $(A\bs B)\bs C \supseteq (A\bs C)\bs (B\bs C)$ | |
\end{enumerate} | |
\paragraph{Subclaim 1:} $(A\bs B)\bs C \subseteq (A\bs C)\bs (B\bs C)$ | |
Let $x$ be an arbitrary element in $(A\bs B)\bs C$. | |
It follows that $x\in A\bs B$ and $x \not \in C$. | |
Since $x \in A\bs B$, it follows that $x \in A$ and $x \not \in B$.\\ | |
Since we know $x \in A$ and $x \not \in C$, it follows that $x \in A \bs C$. | |
Since we know that $x \not \in B$, it follows that $x \not \in B\bs C$.\\ | |
Because $x \in A\bs C$ and $x \not \in B \bs C$, we have shown $x \in (A \bs C) \bs (B \bs C)$. | |
Therefore, since $x$ was an arbitrary element in $(A\bs B)\bs C)$, we have shown that $(A\bs B)\bs C \subseteq (A\bs C)\bs (B\bs C)$. Subclaim 1 is true. | |
\paragraph{Subclaim 2:} $(A\bs B)\bs C \supseteq (A\bs C)\bs (B\bs C)$ | |
[This is where we would prove that subclaim 2 is true. I leave it as an exercise for the reader!] | |
\paragraph{} | |
Because subclaims 1 and 2 are true, our original claim is true. | |
\end{document} |
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