[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}\\\\ 4x+5y=0\implies 5y=-4x\implies y=-\cfrac{4}{5}x\qquad \boxed{k=-\cfrac{4}{5}}\qquad \text{\Large\checkmark}[/tex]
