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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. Homework Help ✎

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

Differentiate:

The equation uses the radius, but the question asks about the diameter.