Qualification > Math

help needed in maths, Diff eqn.

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immortal:
Hi,
i'd appriate if any1 cud solve this doubt, found in O/N09  P31


   In a model of the expansion of a sphere of radius r cm, it is assumed that, at time t seconds after the
      start, the rate of increase of the surface area of the sphere is proportional to its volume.  When t = 0,
                  dr
      r = 5 and -      = 2.
                  dt

       (i)  Show that r satis?es the differential equation

                                                      dr          2
                                                       - = 0.08r  .                                                   
                                                      dt
                                                                                                                                        2
            [The surface area A and volume V of a sphere of radius r are given by the formulae A = 4(Pi)r  ,
                 4        3
            V = -  (Pi)r  .]
                 3

astarmathsandphysics:
Home in half hour

astarmathsandphysics:
   In a model of the expansion of a sphere of radius r cm, it is assumed that, at time t seconds after the
      start, the rate of increase of the surface area of the sphere is proportional to its volume.  When t = 0,
                  dr
      r = 5 and -      = 2.
                  dt

       (i)  Show that r satis?es the differential equation

                                                      dr          2
                                                       - = 0.08r  .                                                  
                                                      dt
                                                                                                                                        2
            [The surface area A and volume V of a sphere of radius r are given by the formulae A = 4(Pi)r  ,
                 4        3
            V = -  (Pi)r  .]
                 3

If the rate of increase is proportional to the suraface area





hence

immortal:
Thanks man! :)
Helped a lot..

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