\section{Introduction}

\begin{definition}[\textbf{Parallelogram Law for Vector Addition}]
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	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{definition}

\begin{definition}
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	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}