Let $x=x(y)$ be the solution of the differential equation
$2(y+2) \log _{e}(y+2) d x+\left(x+4-2 \log _{e}(y+2)\right) d y=0, y>-1$
with $x\left(e^{4}-2\right)=1$. Then $x\left(e^{9}-2\right)$ is equal to :
Solution
$$
\begin{aligned}
& 2(y+2) \ln (y+2) d x+(x+4-2 \ln (y+2)) d y=0 \\\\
& 2 \ln (y+2)+(x+4-2 \ln (y+2)) \frac{1}{y+2} \cdot \frac{d y}{d x}=0 \\\\
& \text { let, } \ln (y+2)=t \\\\
& \frac{1}{y+2} \cdot \frac{d y}{d x}=\frac{d t}{d x} \\\\
& 2 t+(x+4-2 t) \cdot \frac{d t}{d x}=0 \\\\
& (x+4-2 t) \frac{d t}{d x}=-2 t \\\\
& \frac{d x}{d t}=\frac{2 t-4-x}{2 t} \\\\
& \frac{d x}{d t}+\frac{x}{2 t}=\frac{2 t-4}{2 t}
\end{aligned}
$$
<br/><br/>$$
\begin{aligned}
& x \cdot t^{1 / 2}=\int \frac{2 t-4}{2 t} \cdot t^{1 / 2} \cdot d t \\\\
& x \cdot t^{1 / 2}=\int\left(t^{1 / 2}-\frac{2}{t^{1 / 2}}\right) \cdot d t \\\\
& =\frac{t^{\frac{3}{2}}}{\frac{3}{2}}-2 \cdot \frac{t^{\frac{1}{2}}}{\frac{1}{2}}+C \\\\
& x \cdot t^{\frac{1}{2}}=\frac{2 t^{\frac{3}{2}}}{3}-4 \mathrm{t}^{\frac{1}{2}}+C \\\\
& x=\frac{2}{3} \cdot \mathrm{t}-4+\mathrm{C} \cdot \mathrm{t}^{\frac{-1}{2}} \\\\
& x=\frac{2}{3} \ln (\mathrm{y}+2)-4+C \cdot(\ln (\mathrm{y}+2))^{\frac{-1}{2}}
\end{aligned}
$$
<br/><br/>Put $y=e^4-2, x=1$
<br/><br/>$$
\begin{aligned}
& 1=\frac{2}{3} \times 4-4+C \times \frac{1}{2} \\\\
& \frac{C}{2}=5-\frac{8}{3}=\frac{7}{3} \\\\
& C=\frac{14}{3} \\\\
& x=\frac{2}{3} \times 9-4+\frac{14}{3} \times \frac{1}{3} \\\\
& =2+\frac{14}{9} \\\\
& =\frac{32}{9}
\end{aligned}
$$
About this question
Subject: Mathematics · Chapter: Differential Equations · Topic: Linear Differential Equations
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