If $y=y(x)$ is the solution curve of the differential equation $$\left(x^2-4\right) \mathrm{d} y-\left(y^2-3 y\right) \mathrm{d} x=0, x>2, y(4)=\frac{3}{2}$$ and the slope of the curve is never zero, then the value of $y(10)$ equals :
Solution
<p>$$\begin{aligned}
& \left(x^2-4\right) d y-\left(y^2-3 y\right) d x=0 \\
& \Rightarrow \int \frac{d y}{y^2-3 y}=\int \frac{d x}{x^2-4} \\
& \Rightarrow \frac{1}{3} \int \frac{y-(y-3)}{y(y-3)} d y=\int \frac{d x}{x^2-4} \\
& \Rightarrow \frac{1}{3}(\ln |y-3|-\ln |y|)=\frac{1}{4} \ln \left|\frac{x-2}{x+2}\right|+C \\
& \Rightarrow \frac{1}{3} \ln \left|\frac{y-3}{y}\right|=\frac{1}{4} \ln \left|\frac{x-2}{x+2}\right|+C
\end{aligned}$$</p>
<p>$$\begin{aligned}
& \text { At } \mathrm{x}=4, \mathrm{y}=\frac{3}{2} \\
& \therefore \mathrm{C}=\frac{1}{4} \ln 3 \\
& \therefore \frac{1}{3} \ln \left|\frac{\mathrm{y}-3}{\mathrm{y}}\right|=\frac{1}{4} \ln \left|\frac{\mathrm{x}-2}{\mathrm{x}+2}\right|+\frac{1}{4} \ln (3) \\
& \text { At } \mathrm{x}=10 \\
& \frac{1}{3} \ln \left|\frac{\mathrm{y}-3}{\mathrm{y}}\right|=\frac{1}{4} \ln \left|\frac{2}{3}\right|+\frac{1}{4} \ln (3) \\
& \ln \left|\frac{\mathrm{y}-3}{\mathrm{y}}\right|=\ln 2^{3 / 4}, \forall \mathrm{x}>2, \frac{\mathrm{dy}}{\mathrm{dx}}<0 \\
& \text { as } \mathrm{y}(4)=\frac{3}{2} \Rightarrow \mathrm{y} \in(0,3) \\
& -\mathrm{y}+3=8^{1 / 4} \cdot \mathrm{y} \\
& \mathrm{y}=\frac{3}{1+8^{1 / 4}}
\end{aligned}$$</p>
About this question
Subject: Mathematics · Chapter: Differential Equations · Topic: Order and Degree
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