[tex] \bf \qquad \qquad \textit{direct proportional variation}\\\\\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby \begin{array}{llll}k=constant\ of\\\qquad variation\end{array}\\\\[-0.35em]\rule{34em}{0.25pt} [/tex]
[tex] \bf \begin{array}{|cc|ll}
\cline{1-2}
\stackrel{x}{tomatoes~lbs}&\stackrel{y}{price}\\
\cline{1-2}
1&2\\
2&4\\
3&6\\
4&8
\\
\cline{1-2}
\end{array}~\hspace{5em}
\begin{array}{llll}
\textit{we know }
\begin{cases}
x=2\\
y=4
\end{cases}\implies 4=k2
\\\\\\
\cfrac{4}{2}=k\implies \blacktriangleright 2=k \blacktriangleleft
\end{array}
\\\\[-0.35em]
\rule{34em}{0.25pt}\\\\
~\hfill y=2x~\hfill [/tex]
