JEE MAIN - Mathematics (2022 - 25th July Evening Shift - No. 19)
Let $$y=y(x)$$ be the solution of the differential equation
$$\frac{d y}{d x}=\frac{4 y^{3}+2 y x^{2}}{3 x y^{2}+x^{3}}, y(1)=1$$.
If for some $$n \in \mathbb{N}, y(2) \in[n-1, n)$$, then $$n$$ is equal to _____________.
Explanation
$${{dy} \over {dx}} = {y \over x}{{(4{y^2} + 2{x^2})} \over {(3{y^2} + {x^2})}}$$
Put $$y = vx$$
$$ \Rightarrow {{dy} \over {dx}} = v + x{{dv} \over {dx}}$$
$$ \Rightarrow v + x{{dv} \over {dx}} = {{v(4{v^2} + 2)} \over {(3{v^2} + 1)}}$$
$$ \Rightarrow x{{dv} \over {dx}} = v\left( {{{(4{v^2} + 2 - 3{v^2} - 1)} \over {3{v^2} + 1}}} \right)$$
$$ \Rightarrow \int {(3{v^2} + 1){{dv} \over {{v^3} + v}} = \int {{{dx} \over x}} } $$
$$ \Rightarrow \ln |{v^3} + v| = \ln x + c$$
$$ \Rightarrow \ln \left| {{{\left( {{y \over x}} \right)}^3} + \left( {{y \over x}} \right)} \right| = \ln x + C$$
$$ \downarrow \,y(1) = 1$$
$$ \Rightarrow C = \ln 2$$
$$\therefore$$ for $$y(2)$$
$$\ln \left( {{{{y^3}} \over 8} + {y \over 2}} \right) = 2\ln 2 \Rightarrow {{{y^3}} \over 8} + {y \over 2} = 4$$
$$ \Rightarrow [y(2)] = 2$$
$$ \Rightarrow n = 3$$
Comments (0)
