Chapter 2 - The Derivative - 2.5 Derivatives of Trigonometric Functions - Exercises Set 2.5 - Page 151: 33

$1.75 m/deg$

Using the definition of tangent we can write D as a function of theta, and then derivate with respect to theta as in the image below. If you don't remember that $\frac{d}{dx} [tan x]= sec^{2}x$, you can simply write tangent as $\frac{sin x}{cos x}$ and use the division rule (or product rule, thinking about $sin x\times\frac{1}{cos x}$). At the end, the answer is in meters/radian, so we need to convert to degree, remembering that $\pi$ radians are equivalent to 180°, for example.