Wrote out Chapter 1 theorems and definitions

chapter-1/introduction.tex

@@ -1 +1,11 @@
 \section{Introduction}
+
+\begin{theorem}[\textbf{Parallelogram Law for Vector Addition}]
+	\hfill\\
+	The sum of two vectors $x$ and $y$ that act at the same point $P$ is the vector beginning at $P$ that is represented by the diagonal of parallelogram having $x$ and $y$ as adjacent sides.
+\end{theorem}
+
+\begin{definition}
+	\hfill\\
+	Two nonzero vectors $x$ and $y$ are called \textbf{parallel} if $y=tx$ for some nonzero real number $t$. (Thus nonzero vectors having the same or opposite directions are parallel.)
+\end{definition}