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Mathematics · 2022 · 3 marks
Differential EquationsMethods of Solving First Order, First Degree Differential Equations3 marksApplyshort answer
CBSE 2022 · Region 2 · Set 3 · Q9
Solve the following differential equation: (1+e^y / x) d y+e^y / x(1-y/x) d x=0
Solve the following differential equation : \[\left(1+e^{y / x}\right) d y+e^{y / x}\left(1-\frac{y}{x}\right) d x=0 \]
Marking-scheme solution
Given differential equation can be written as
\[\frac{d y}{d x}=\frac{e^{y / x}\left(\dfrac{y}{x}-1\right)}{1+e^{y / x}}
\]
put \(\displaystyle y=v x\) and \(\displaystyle \frac{d y}{d x}=v+x \frac{d v}{d x}\), we get
\[v+x \frac{d v}{d x}=\frac{e^{v}(v-1)}{1+e^{v}}
\]
\(\displaystyle x \frac{d v}{d x}=\frac{-e^{v}-v}{1+e^{v}}\)
\[\int \frac{\left(e^{v}+1\right)}{e^{v}+v} d v=-\int \frac{d x}{x}
\]
\(\displaystyle \log \left|e^{v}+v\right|=-\log |x|+\log C\)
\[e^{v}+v=\frac{C}{x}
\]
\(\displaystyle e^{y / x}+\frac{y}{x}=\frac{C}{x} \quad\) or \(\displaystyle x e^{y / x}+y=C\)
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CBSE Class 12 Mathematics past-paper question from the 2022 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.