June 2013 p33 q8
2278
The variables x and t satisfy the differential equation
\(t \frac{dx}{dt} = \frac{k - x^3}{2x^2}\),
for \(t > 0\), where \(k\) is a constant. When \(t = 1, x = 1\) and when \(t = 4, x = 2\).
(i) Solve the differential equation, finding the value of \(k\) and obtaining an expression for \(x\) in terms of \(t\). [9]
(ii) State what happens to the value of \(x\) as \(t\) becomes large. [1]
Solution
(i) Separate variables and integrate:
\(\int 2x^2 \, dx = \int \frac{k - x^3}{t} \, dt\)
Integrate to obtain:
\(\ln t = -\frac{2}{3} \ln(k - x^3) + \frac{2}{3} \ln(k - 1)\)
Use initial conditions \(t = 1, x = 1\) and \(t = 4, x = 2\) to solve for \(k\):
\(\ln 4 = -\frac{2}{3} \ln(k - 8) + \frac{2}{3} \ln(k - 1)\)
Solve for \(k\) to obtain \(k = 9\).
Substitute \(k = 9\) to find \(x\):
\(x = (9 - 8t^{\frac{3}{2}})^{\frac{1}{3}}\)
(ii) As \(t \to \infty\), \(x \to 9^{\frac{1}{3}}\).
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