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2008 Paper 3 Q6
D: 1700.0 B: 1500.0

In this question, \(p\) denotes \(\dfrac{\d y}{\d x}\,\).

  1. Given that \[ y=p^2 +2 xp\,, \] show by differentiating with respect to \(x\) that \[ \frac{\d x}{\d p} = -2 - \frac {2x} p . \] Hence show that \(x = -\frac23p +Ap^{-2}\,,\) where \(A\) is an arbitrary constant. Find \(y\) in terms of \(x\) if \(p=-3\) when \(x=2\).
  2. Given instead that \[ y=2xp +p \ln p\,,\] and that \(p=1\) when \(x=-\frac14\), show that \(x=-\frac12 \ln p - \frac14\,\) and find \(y\) in terms of \(x\).