CBSE Class 12 Mathematics Differential Equations Q #1791
COMPETENCY BASED
APPLY
3 Marks 2026 AISSCE(Board Exam) SA
Find a particular solution of the differential equation $(x+1)\frac{dy}{dx}=2 e^{-y}-1$ given that $y=0$ when $x=0$.

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Detailed Solution

Step 1: Separate the variables

The given differential equation is $(x+1)\frac{dy}{dx} = 2e^{-y} - 1$. Rearranging the terms to separate $x$ and $y$, we get: $$ \frac{dy}{2e^{-y} - 1} = \frac{dx}{x+1} $$ Multiplying the numerator and denominator of the left side by $e^y$: $$ \frac{e^y dy}{2 - e^y} = \frac{dx}{x+1} $$

Step 2: Integrate both sides

Integrate both sides: $$ \int \frac{e^y}{2 - e^y} dy = \int \frac{1}{x+1} dx $$ For the left integral, let $u = 2 - e^y$, then $du = -e^y dy$. The integral becomes: $$ -\int \frac{du}{u} = \ln|x+1| + C $$ $$ -\ln|2 - e^y| = \ln|x+1| + C $$

Step 3: Apply initial conditions

Given $y=0$ when $x=0$: $$ -\ln|2 - e^0| = \ln|0+1| + C $$ $$ -\ln|2 - 1| = \ln(1) + C $$ $$ -\ln(1) = 0 + C \implies C = 0 $$

Step 4: Final expression

Substituting $C=0$ back into the equation: $$ -\ln|2 - e^y| = \ln|x+1| $$ $$ \ln|2 - e^y|^{-1} = \ln|x+1| $$ $$ \frac{1}{2 - e^y} = x + 1 $$ $$ 2 - e^y = \frac{1}{x+1} \implies e^y = 2 - \frac{1}{x+1} = \frac{2x+1}{x+1} $$ $$ y = \ln\left(\frac{2x+1}{x+1}\right) $$

Final Answer: y = \ln\left(\frac{2x+1}{x+1}\right)

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because it requires the student to utilize the method of separation of variables and integration techniques to solve a specific differential equation.
Knowledge Dimension: PROCEDURAL
Justification: The question tests the mastery of a specific algorithmic process (solving differential equations) rather than just factual recall.
Syllabus Audit: In the context of CBSE Class 12, this is classified as COMPETENCY. This aligns with the NCERT curriculum for Differential Equations, focusing on the application of integration to find particular solutions.