Solve $2(y + 3) - xy\frac{{dy}}{{dx}} = 0,$ given that $y(1) = - 2$.
Solve $2(y + 3) - xy\frac{{dy}}{{dx}} = 0,$ given that $y(1) = - 2$.
Official Solution
Given that, $2(y + 3) - xy\frac{{dy}}{{dx}} = 0$
$\Rightarrow$ $2(y + 3) = xy\frac{{dy}}{{dx}}$
$\Rightarrow$ $2\frac{{dx}}{x} = \left( {\frac{y}{{y + 3}}} \right)dy$
$\Rightarrow$ $2 \cdot \frac{{dx}}{x} = \left( {\frac{{y + 3 - 3}}{{y + 3}}} \right)dy$
$\Rightarrow$ $2 \cdot \frac{{dx}}{x} = \left( {1 - \frac{3}{{y + 3}}} \right)dy$
On integrating both sides,
we get
$2\log x = y - 3\log (y + 3) + C$
……(i)
When $x = 1$ and $y = - 2,$
then
$2\log 1 = - 2 - 3\log ( - 2 + 3) + C$
$\Rightarrow$ $2 \cdot 0 = - 2 - 3 \cdot 0 + C$
$\Rightarrow$ $C = 2$
On substituting the value of $C$ in Eq.
(i), we get $2\log x = y - 3\log (y + 3) + 2$
$\Rightarrow$ $2\log x + 3\log (y + 3) = y + 2$
$\Rightarrow$ $\log {x^2} + \log {(y + 3)^3} = (y + 2)$
$\Rightarrow$ $\log {x^2}{(y + 3)^3} = y + 2$
$\Rightarrow$ ${x^2}{(y + 3)^3} = {e^{y + 2}}$
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