Class CBSE Class 12 Mathematics Differential Equations Q #1821
COMPETENCY BASED
APPLY
5 Marks 2026 AISSCE(Board Exam) LA
Solve the differential equation $y~e^{y}dx = (y^{3} + 2x~e^{y}) dy$, when $y(0)=1$.

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Detailed Solution

Step 1: Rearrange the equation

The given equation is $y e^{y} dx = (y^{3} + 2x e^{y}) dy$. We can rewrite this as a linear differential equation in terms of $x$ by dividing by $dy$ and $y e^{y}$:

$$\frac{dx}{dy} = \frac{y^{3} + 2x e^{y}}{y e^{y}}$$ $$\frac{dx}{dy} = \frac{y^{2}}{e^{y}} + \frac{2x}{y}$$

Step 2: Standard Linear Form

Rearranging the terms to fit the form $\frac{dx}{dy} + P(y)x = Q(y)$:

$$\frac{dx}{dy} - \left(\frac{2}{y}\right)x = y^{2} e^{-y}$$

Step 3: Find the Integrating Factor (IF)

Here, $P(y) = -\frac{2}{y}$. The integrating factor is:

$$IF = e^{\int P(y) dy} = e^{\int -\frac{2}{y} dy} = e^{-2 \ln|y|} = e^{\ln(y^{-2})} = \frac{1}{y^{2}}$$

Step 4: Solve the equation

Multiply the linear equation by the IF and integrate:

$$\frac{1}{y^{2}} \frac{dx}{dy} - \frac{2}{y^{3}} x = e^{-y}$$ $$\frac{d}{dy} \left( \frac{x}{y^{2}} \right) = e^{-y}$$ $$\frac{x}{y^{2}} = \int e^{-y} dy = -e^{-y} + C$$ $$x = y^{2} (C - e^{-y})$$

Step 5: Apply initial condition

Given $y(0) = 1$, substitute $x=0$ and $y=1$:

$$0 = 1^{2} (C - e^{-1}) \implies C = e^{-1} = \frac{1}{e}$$ $$x = y^{2} \left( \frac{1}{e} - e^{-y} \right)$$

Final Answer: x = y^{2} (e^{-1} - e^{-y})

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because students must recognize the non-standard form of the differential equation and transform it into a standard linear form to apply the integrating factor method.
Knowledge Dimension: PROCEDURAL
Justification: The problem requires a specific sequence of algorithmic steps (rearrangement, identifying P(y), calculating IF, integration, and constant evaluation).
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. It tests the student's ability to manipulate variables in a differential equation beyond simple variable separation.