Differential Equations
Formation and Solution of Differential Equations
GRB_1000_MCQ
Grade Class 12
Question:
A curve passes through $(-2, -2)$ and its slope at the point $(x, y)$ is given by $\dfrac{1}{x\sqrt{x^2-1}}$.
Which of the following points the curve also passes through?
$\left(\dfrac{-2}{\sqrt{3}}, \dfrac{-\pi}{6} - 2\right)$
$\left(\dfrac{-2}{\sqrt{3}}, \dfrac{\pi}{6} - 2\right)$
$\left(-\sqrt{2}, \dfrac{-\pi}{12} - 2\right)$
$\left(-\sqrt{2}, \dfrac{\pi}{12} - 2\right)$
Step-by-Step Solution
Key Concept: The key idea is to solve the first-order differential equation $\frac{dy}{dx} = \frac{1}{x\sqrt{x^2-1}}$ by direct integration, utilizing the standard integral for the inverse secant function. The constant of integration is then determined by applying the given initial condition.
Step 1: Set up the differential equation.
The slope at the point $(x, y)$ is given by:
$$\frac{dy}{dx} = \frac{1}{x\sqrt{x^2-1}}$$
Step 2: Integrate to find the general solution.
The curve passes through $(-2, -2)$, which implies $x < -1$.
For $x < -1$, let $u = -x$. Then $u > 1$.
The derivative of $\sec^{-1}(u)$ with respect to $u$ is $\frac{1}{u\sqrt{u^2-1}}$.
Using the chain rule, the derivative of $\sec^{-1}(-x)$ with respect to $x$ is:
$$\frac{d}{dx}[\sec^{-1}(-x)] = \frac{1}{(-x)\sqrt{(-x)^2-1}} \cdot (-1) = \frac{1}{-x\sqrt{x^2-1}} \cdot (-1) = \frac{1}{x\sqrt{x^2-1}}$$
Therefore, the integral of $\frac{1}{x\sqrt{x^2-1}}$ for $x < -1$ is:
$$y = \sec^{-1}(-x) + C$$
Step 3: Apply the initial condition.
The curve passes through the point $(-2, -2)$. Substitute $x = -2$ and $y = -2$ into the equation:
$$-2 = \sec^{-1}(-(-2)) + C$$
$$-2 = \sec^{-1}(2) + C$$
The principal value of $\sec^{-1}(2)$ is $\frac{\pi}{3}$.
$$-2 = \frac{\pi}{3} + C$$
Solving for $C$:
$$C = -2 - \frac{\pi}{3}$$
Thus, the equation of the curve is:
$$y = \sec^{-1}(-x) - 2 - \frac{\pi}{3}$$
Step 4: Verify points on the curve.
To determine other points the curve passes through, substitute the x-coordinates into the curve's equation.
For $x = -\frac{2}{\sqrt{3}}$:
$$-x = \frac{2}{\sqrt{3}}$$
$$y = \sec^{-1}\left(\frac{2}{\sqrt{3}}\right) - 2 - \frac{\pi}{3}$$
The principal value of $\sec^{-1}\left(\frac{2}{\sqrt{3}}\right)$ is $\frac{\pi}{6}$.
$$y = \frac{\pi}{6} - 2 - \frac{\pi}{3}$$
$$y = \frac{\pi - 2\pi}{6} - 2$$
$$y = -\frac{\pi}{6} - 2$$
Therefore, the curve passes through $\left(-\frac{2}{\sqrt{3}}, -\frac{\pi}{6} - 2\right)$.
For $x = -\sqrt{2}$:
$$-x = \sqrt{2}$$
$$y = \sec^{-1}(\sqrt{2}) - 2 - \frac{\pi}{3}$$
The principal value of $\sec^{-1}(\sqrt{2})$ is $\frac{\pi}{4}$.
$$y = \frac{\pi}{4} - 2 - \frac{\pi}{3}$$
$$y = \frac{3\pi - 4\pi}{12} - 2$$
$$y = -\frac{\pi}{12} - 2$$
Therefore, the curve passes through $\left(-\sqrt{2}, -\frac{\pi}{12} - 2\right)$.
Correct Answer: 2, 3