Class CBSE Class 12 Mathematics Differential Equations Q #918
KNOWLEDGE BASED
APPLY
3 Marks 2023 SA
Find the general solution of the differential equation : $\frac{d}{dx}(xy^{2})=2y(1+x^{2})$

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Step-by-Step Solution

Given differential equation: $\frac{d}{dx}(xy^{2})=2y(1+x^{2})$

Differentiate the left side using the product rule: $y^{2} + x(2y\frac{dy}{dx}) = 2y(1+x^{2})$

Rearrange the equation: $y^{2} + 2xy\frac{dy}{dx} = 2y + 2yx^{2}$

Divide by $y$ (assuming $y \neq 0$): $y + 2x\frac{dy}{dx} = 2 + 2x^{2}$

Isolate the $\frac{dy}{dx}$ term: $2x\frac{dy}{dx} = 2 + 2x^{2} - y$

Rewrite the equation: $\frac{dy}{dx} = \frac{2 + 2x^{2} - y}{2x}$

This differential equation is not easily separable or linear. Let's try a different approach from the beginning.

Given: $\frac{d}{dx}(xy^{2})=2y(1+x^{2})$

Integrate both sides with respect to $x$: $\int \frac{d}{dx}(xy^{2}) dx = \int 2y(1+x^{2}) dx$

$xy^{2} = \int 2y(1+x^{2}) dx$

This approach requires us to express $y$ as a function of $x$ inside the integral, which is not straightforward. Let's go back to the differentiated form.

From $y^{2} + 2xy\frac{dy}{dx} = 2y(1+x^{2})$, we can rewrite it as:

$y^{2} + 2xy\frac{dy}{dx} = 2y + 2yx^{2}$

Divide throughout by $y$: $y + 2x\frac{dy}{dx} = 2 + 2x^{2}$

Rearrange: $2x\frac{dy}{dx} = 2x^{2} - y + 2$

Let's try another approach. Rewrite the original equation as $d(xy^2) = 2y(1+x^2)dx$.

Then, $xy^2 = \int 2y(1+x^2) dx$. This is difficult to solve directly.

Let's consider the original equation again: $\frac{d}{dx}(xy^{2})=2y(1+x^{2})$.

Expanding the derivative: $y^2 + 2xy \frac{dy}{dx} = 2y + 2yx^2$.

Dividing by $y$ (assuming $y \neq 0$): $y + 2x \frac{dy}{dx} = 2 + 2x^2$.

Rearranging: $2x \frac{dy}{dx} = 2x^2 - y + 2$.

This is not a separable equation in its current form.

Let's try to rearrange the equation as follows:

$\frac{d(xy^2)}{dx} = 2y(1+x^2)$

$d(xy^2) = 2y(1+x^2)dx$

This form is still difficult to integrate directly.

Let's go back to $y + 2x\frac{dy}{dx} = 2 + 2x^{2}$.

Rearranging, we get $2x\frac{dy}{dx} = 2x^2 - y + 2$.

This is a first-order differential equation, but it's not linear or separable.

Let's try to find an integrating factor. Rewrite the equation as:

$\frac{dy}{dx} + \frac{1}{2x}y = x + \frac{1}{x}$

The integrating factor is $e^{\int \frac{1}{2x} dx} = e^{\frac{1}{2} \ln x} = e^{\ln \sqrt{x}} = \sqrt{x}$.

Multiply the equation by the integrating factor:

$\sqrt{x} \frac{dy}{dx} + \frac{1}{2\sqrt{x}}y = x\sqrt{x} + \frac{1}{\sqrt{x}}$

$\frac{d}{dx}(y\sqrt{x}) = x\sqrt{x} + \frac{1}{\sqrt{x}}$

Integrate both sides with respect to $x$:

$\int \frac{d}{dx}(y\sqrt{x}) dx = \int (x\sqrt{x} + \frac{1}{\sqrt{x}}) dx$

$y\sqrt{x} = \int (x^{3/2} + x^{-1/2}) dx$

$y\sqrt{x} = \frac{x^{5/2}}{5/2} + \frac{x^{1/2}}{1/2} + C$

$y\sqrt{x} = \frac{2}{5}x^{5/2} + 2x^{1/2} + C$

Divide by $\sqrt{x}$:

$y = \frac{2}{5}x^2 + 2 + \frac{C}{\sqrt{x}}$

Correct Answer: $y = \frac{2}{5}x^2 + 2 + \frac{C}{\sqrt{x}}$

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student needs to apply their knowledge of differential equations and integration techniques to solve the given problem. They must use specific procedures to separate variables and integrate.
Knowledge Dimension: PROCEDURAL
Justification: The question requires the student to follow a specific procedure to solve the differential equation, including separation of variables and integration. The focus is on the method rather than just recalling facts or understanding concepts.
Syllabus Audit: In the context of CBSE Class 12, this is classified as KNOWLEDGE. The question directly tests the student's ability to solve differential equations, a standard topic covered in the textbook.