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7-48.

Alien spaceshipAn alien uses mysterious powers to make crop circles in a Nebraska wheat field. If the radius of a crop circle increases at the rate of ft/min, calculate the rate at which the circle’s area is increasing when the radius is feet. Homework Help ✎

Start with the geometric equation of the area within a circle. There is no Calculus involved in this step.

Do NOT substitute a value for r, because the r changes. That is where Calculus comes in...

Implicitly differentiate with respect to time, t.