2021 MI P1 Q3

2021 MI P1 Q3

7 marks
Prelims
  1. It is given that \({{x}^{2}}\frac{\mathrm{d}y}{\mathrm{d}x}-3xy+4=0\). Using the substitution \(y=u{{x}^{3}}\), show that the differential equation can be transformed to \(\frac{\mathrm{d}u}{\mathrm{d}x}=\frac{c}{{{x}^{5}}}\), where \(c\) is a constant to be determined.[3]
  2. Hence given that \(y=3\) when \(x=1\), solve the differential equation \({{x}^{2}}\frac{\mathrm{d}y}{\mathrm{d}x}-3xy+4=0\) to find \(y\)in terms of \(x\).[4]

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Answer:(i) \(\frac{\mathrm{d}u}{\mathrm{d}x} = -\frac{4}{x^5}\); (ii) \(y = \frac{1}{x} + 2x^3\)

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