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math please help

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A ruptured oil tanker causes a circular oil slick on the surface of the ocean. When its radius is 150 meters, the radius of the slick is expanding at 0.1 m/min, and its thickness is .02 m. At that moment:
a. How fast is the area of the slick expanding?
b. If the circular slick has the same thickness as everywhere, and the volume of oil spilled remains fixed, how fast is the thickness of the slick decreasing?

  • math please help - ,

    Area = πr^2
    dA/dt = 2πr(dr/dt)
    for the given data
    dA/dt = 2π(150)(.1) m^2/min
    = 94.25 m^2/min

    Volume = Areax height = Ah
    dV/dt = A(dh/dt) + h(dA/dt)

    so when r = 150, A = π(150)^2 = 22500π
    dV/dt = 0 (it remains constant) and
    h = .02

    0 = 22500π(dh/dt) + .02(94.25)
    dh/dt = - .02(94.25)/22500π
    = - .000026667 m/min

    check my arithmetic

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