Consider the differential equation : $$ \frac{d y}{d x}=\frac{y^3}{2\left(x y^2-x^2\right)} $$ Statement-1:…

Consider the differential equation : $$ \frac{d y}{d x}=\frac{y^3}{2\left(x y^2-x^2\right)} $$ Statement-1: The substitution $z=y^2$ transforms the above equation into a first order homogenous differential equation. Statement-2: The solution of this differential equation is $y^2 e^{-y^2} / x=C$.
  1. Both statements are false.
  2. Statement-1 is true and statement- 2 is false.
  3. Statement-1 is false and statement-2 is true.
  4. Both statements are true.

Solution

Given differential equation is $ \frac{d y}{d x}=\frac{y^3}{2\left(x y^2-x^2\right)} $ By substituting $z=y^2$, we get diff. eqn. as $ \frac{d z}{d x}=\frac{2 z^2}{2\left(x z-x^2\right)}=\frac{z^2}{x z-x^2} $ Now, $\frac{d x}{d z}=\frac{x}{z}-\frac{x^2}{z^2}=\frac{x}{z}\left[1-\frac{x}{z}\right] \approx \mathrm{F}\left(\frac{x}{z}\right)$ Hence, statement-1 is true. Now, $y^2 e^{-y^2 / x}=\mathrm{C}$ satisfies the given diff. equation $\therefore$ It is the solution of given diff. equation. Thus, statement $-2$ is also true

Asked in: JEE Main 2013 (22 Apr Online)

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