Calculus

The median value of a home in a particular market is decreasing exponentially. If the value of a home
was initially $240,000, then its value two years later is $235,000.

a) Write a differential equation that models this situation. Let V represent the value of the home (in thousands of dollars) and t represent the number of years since its value was $240,000.

For this, I got dV/dt = kV but I'm not positive it's correct. I also need to solve for the particular solution in terms of V and t and I'm not sure how to do that.

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asked by Jamie
  1. dV/dt = k V

    dV/V = k dt

    ln V = k t + C

    e^ln V = V definition of ln

    V = e^(kt + C)
    C is arbitrary so far
    V = e^kt * e^C
    = C e^kt since e^c could be any old C
    Now that is general solution. Now put in t = 0
    at t = 0 e^kt = e^0 = 1
    so
    240,000 = C e^0 = C
    so
    V = 240,000 e^kt
    Now if t = 2, V = 235,000
    235 = 240 e^2k

    e^2k = .979
    ln e^2k = 2k = -.02105
    so
    k = - .010526
    and
    V = 240,000 e^-.010526 t

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    posted by Damon
  2. If the question says to write the differential equation "in thousands of dollars" would the particular solution be written as V(t)=240e^((-.010526)(t)) instead of V = 240,000 e^-.010526 t ?

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    posted by Jamie
  3. sure, in fact I did it with 235 = 240 e^2k

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    posted by Damon
  4. The last question for this problem asks to find the relative rate of change in the home's value at any time t. How would I do that?

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    posted by Jamie
  5. You have dV/dt = k V
    and you know that k = -.010526

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    posted by Damon
  6. So would it be -.010526 = (dV/dt)/V ?

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    posted by Jamie
  7. I suppose :)

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    posted by Damon

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