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10-74.

Mariah Mariah walks towards a wall with a flashlight beaming straight ahead. When she is feet away from the wall, the radius of the circle of light is feet. If she walks at a rate of ft/sec, calculate 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.