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June 2016 p31 q4
2277
The variables x and y satisfy the differential equation
\(x \frac{dy}{dx} = y(1 - 2x^2)\),
and it is given that \(y = 2\) when \(x = 1\). Solve the differential equation and obtain an expression for y in terms of x in a form not involving logarithms.
Solution
First, separate the variables:
\(\frac{1}{y} dy = \frac{1 - 2x^2}{x} dx\)
Integrate both sides:
\(\int \frac{1}{y} dy = \int \frac{1 - 2x^2}{x} dx\)
This gives:
\(\ln y = \ln x - x^2 + C\)
Use the initial condition \(y = 2\) when \(x = 1\) to find \(C\):