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    Sphere of radius R melts such that the radius after time t is r and the rate of change is given by:

    dr/dt = -r

    If T is the time taken for the radius of the sphere to decrease to (1/4)R, show that T = ln4

    -----

    When I integrate I get

    ln r = -t + lnA

    but don't know what to do from there (if that's right)

    Thanks.
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    You know that when t = 0 then r = R.

    Use this to solve your constant of integration and you are home.
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    (Original post by Mr M)
    You know that when t = 0 then r = R.

    Use this to solve your constant of integration and you are home.
    Thank you!
 
 
 
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