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Math Help - Calculus applications

  1. #1
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    Calculus applications

    Each edge of a cube is expanding at a rate of 4cm/s. How fast is the volume changing when each edge is 5cm? At what rate is the surface area changing when each edge is 7 cm?
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    Quote Originally Posted by mathamatics112 View Post
    Each edge of a cube is expanding at a rate of 4cm/s. How fast is the volume changing when each edge is 5cm? At what rate is the surface area changing when each edge is 7 cm?
    <br />
\frac{dV}{dt} = (\frac{dV}{dr})(\frac{dr}{dt})

    V = r^3 and \frac{dr}{dt} = 4cm.s^{-1}

    Can you go from there?
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    is that the equation i am using for the first one?
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    Quote Originally Posted by mathamatics112 View Post
    is that the equation i am using for the first one?
    Note, I'm using r for any one side on the cube rather than a radius

    Yeah, it's an application of the chain rule.

    ----

    2. As the surface area is A = 6r^2 and \frac{dr}{dt} = 4cm.s^{-1}

    this will be \frac{dA}{dt} =(\frac{dA}{dr})(\frac{dr}{dt})
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