### Home > CALC > Chapter 10 > Lesson 10.1.6 > Problem10-58

10-58.

Mariah walks toward a wall with a flashlight beaming straight ahead. When she is $5$ feet away, the radius of the circle is $2$ feet. If she walks $0.5$ feet per second, find the rate at which the diameter of the circle of light is changing.

The radius is directly proportional to Mariah's distance from the wall. That is, $d = kr$.

Differentiate: