Points $\displaystyle {A}$ and $\displaystyle {B}$ move along the $\displaystyle {x}$ and $\displaystyle {y}$ axes, respectively, in such a way that the distance $\displaystyle {r}$ (in inches) along the perpendicular from the origin $\displaystyle {O}$ to the line segment $\displaystyle {A}{B}$ remains constant, as demonstrated in the applet below. How fast is the length of OA changing at the instant when OB has length $\displaystyle {3}{r}$ and $\displaystyle {B}$ is moving toward the origin at a rate of $\displaystyle {1.3}{r}$ inches per second? Note: Your answer will be an expression in terms of $\displaystyle {r}$.

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Round any decimals to 4 places.