Question 19 (Differential Equation)

Let $y(x)$ is the solution of the D.E. $y' = y(y-2); y(0)=\alpha,$ then-

(a) $y(x)$ is increasing for all $\alpha$

(b) $y(x)$ is decreasing for all $\alpha$.

(c) $y(x)$ is increasing if $\alpha \in (-\infty, 0) \cup (2, \infty)$

(d) $y(x)$ is decreasing if $\alpha \in (0, 2)$


Explanation: (c) and (d) are correct.

Critical points of $y(x)$ are given by $y'=0$ i.e $y(y-2)=0$. Thus $y=0, y=2$ are critical points.

$\implies \dfrac{dy}{dx} > 0$ if $(-\infty, 0) \cup (2, \infty)$, and $\dfrac{dy}{dx} < 0$ if $y \in (0, 2)$

$\implies$ (c) and (d) are correct.

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