Science:Math Exam Resources/Courses/MATH102/December 2014/Question B 07/Hint 1

Set the angle between the line which connects the runner and the origin and the positive x-axis to be $\theta$, then if the runner is at location $(x,y)$, we have $\tan(\theta )={\frac {y}{x}}$. We know $\theta '=1/10$, so we can use implicit differentiation to relate $\theta '$ with $x'$ and $y'$.