Infection model differential equation solved by partial fractions, with long-term limit

Infection model differential equation solved by partial fractions, with long-term limit

IB Year 6 | Grade 12
18 marks

A disease spreads through a colony of rabbits. Let \(N\) be the number of infected rabbits, in thousands, \(t\) days after the outbreak is first observed.

The spread is modelled by the differential equation \(\frac{\mathrm{d}N}{\mathrm{d}t}=k(-3+4N-N^2)\), where \(k\) is a positive constant and \(t\geq0\).

When \(t=0\), \(1500\) rabbits are infected.

  1. Write \(\frac{1}{-3+4x-x^2}\) as partial fractions of the form \(\frac{A}{x-1}+\frac{B}{3-x}\), where \(A\) and \(B\) are constants.[3]
  2. Hence, show that \(\ln\left(\frac{N-1}{3-N}\right)=2kt-\ln p\), stating the value of the positive constant \(p\).[8]
  3. Express \(N\) in terms of \(k\) and \(t\), in the form \(N=\frac{a+be^{-2kt}}{1+ce^{-2kt}}\), where \(a\), \(b\) and \(c\) are integers.[5]
  4. In the long term, the model predicts that the number of infected rabbits tends to a limiting value, \(L\).
    Find \(L\).[2]

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Answer:(a) \(A=B=\frac12\) (b) \(p=3\) (c) \(N=\frac{3+3e^{-2kt}}{1+3e^{-2kt}}\) (d) \(L=3000\)

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