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Math Help - Expansion problem

  1. #1
    Junior Member
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    Expansion problem

    All edges of a cube are expanding at a rate of 3cm per second. How fast is the surface area changing when each edge is (a) 1cm and (b) 10 cm?


    Not sure how to approach the probme.
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  2. #2
    MHF Contributor alexmahone's Avatar
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    S=6a^2
    \frac{dS}{dt}=6*2a\frac{da}{dt}

    \frac{da}{dt}=3 cm/sec

    \frac{dS}{dt}=6*2a*3
    \frac{dS}{dt}=36a

    a) \frac{dS}{dt}=36*1=36 cm^2/sec

    b) \frac{dS}{dt}=36*10=360 cm^2/sec
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