June 2020 p32 q7
2329
The variables x and y satisfy the differential equation
\(\frac{dy}{dx} = \frac{y-1}{(x+1)(x+3)}\).
It is given that \(y = 2\) when \(x = 0\).
Solve the differential equation, obtaining an expression for \(y\) in terms of \(x\).
Solution
Separate variables:
\(\frac{dy}{y-1} = \frac{dx}{(x+1)(x+3)}\)
Integrate both sides:
\(\int \frac{dy}{y-1} = \int \frac{dx}{(x+1)(x+3)}\)
\(\ln|y-1| = \int \left( \frac{A}{x+1} + \frac{B}{x+3} \right) dx\)
Find \(A\) and \(B\) such that:
\(\frac{1}{(x+1)(x+3)} = \frac{A}{x+1} + \frac{B}{x+3}\)
Solving gives \(A = \frac{1}{2}\) and \(B = -\frac{1}{2}\).
Integrate:
\(\ln|y-1| = \frac{1}{2} \ln|x+1| - \frac{1}{2} \ln|x+3| + C\)
\(\ln|y-1| = \frac{1}{2} \ln\left(\frac{x+1}{x+3}\right) + C\)
Exponentiate both sides:
\(|y-1| = e^C \sqrt{\frac{x+1}{x+3}}\)
Use initial condition \(y = 2\) when \(x = 0\):
\(1 = e^C \sqrt{\frac{1}{3}}\)
\(e^C = \sqrt{3}\)
Thus, \(y - 1 = \sqrt{3} \sqrt{\frac{x+1}{x+3}}\)
\(y = 1 + \sqrt{\frac{3x+3}{x+3}}\)
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