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DIFFERENTIAL EQUATIONS — Maths STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceMathsDIFFERENTIAL EQUATIONS1 Mark
MCQ
Choose the correct answer from the given four options.The general solution of the differential equation $(\text{e}^{\text{x}}+1)\text{ydy}=(\text{y}+1)\text{e}^{\text{x}}$ is:
  • A
    $(\text{y}+1)=\text{k}(\text{e}^{\text{x}}+1)$
  • B
    $\text{y}+1=\text{e}^{\text{x}}+1+\text{k}$
  • ✓
    $\text{y}=\log\left\{\text{k}(\text{y}+1)(\text{e}^{\text{x}}+1)\right\}$
  • D
    $\text{y}=\log\left\{\frac{\text{e}^{\text{x}}+1}{\text{y}+1}\right\}+\text{k}$
✓

Answer

Correct option: C.
$\text{y}=\log\left\{\text{k}(\text{y}+1)(\text{e}^{\text{x}}+1)\right\}$
Given differential equation

$(\text{e}^{\text{x}}+1)\text{ydy}=(\text{y}+1)\text{e}^{\text{x}}\text{dx}$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{\text{e}^{\text{x}}(1+\text{y})}{(\text{e}^{\text{x}}+1)\text{y}}$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{(\text{e}^{\text{x}}+1)\text{y}}{\text{e}^{\text{x}}(1+\text{y})}$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{\text{e}^{\text{x}}\text{y}}{\text{e}^{\text{x}}(1+\text{y})}\frac{\text{y}}{\text{e}^{\text{x}}(1+\text{y})}$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{\text{y}}{1+\text{y}}+\frac{\text{y}}{(1+\text{y})\text{e}^{\text{x}}}$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{\text{y}}{1+\text{y}}\Big(1+\frac{1}{\text{e}^{\text{x}}}\Big)\text{dx}$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{\text{y}}{1+\text{y}}\Big(1+\frac{1}{\text{e}^{\text{x}}}\Big)$

$\Rightarrow\frac{\text{dy}}{\text{dx}}=\frac{\text{y}}{1+\text{y}}\Big(\frac{\text{e}^{\text{x}}+1}{\text{e}^{\text{x}}}\Big)$

$\Rightarrow\Big(\frac{\text{y}}{1+\text{y}}\Big)\text{dy}=\Big(\frac{\text{e}^{\text{x}}}{\text{e}^{\text{x}}+1}\Big)\text{dx}$

On integrating both sides, we get

$\int\frac{\text{y}}{1+\text{y}}\text{dy}=\int\frac{\text{e}^{\text{x}}}{1+\text{e}^{\text{x}}}\text{dx}$

$\Rightarrow\int\frac{1+\text{y}-1}{1+\text{y}}\text{dy}=\int\frac{\text{e}^{\text{x}}}{1+\text{e}^{\text{x}}}\text{dx}$

$\Rightarrow\int1\text{dy}-\int\frac{\text{y}}{1+\text{y}}\text{dy}=\int\frac{\text{e}^{\text{x}}}{1+\text{e}^{\text{x}}}\text{dx}$

$\Rightarrow\text{y}-\log|(1+\text{y})+\log(1+\text{e}^{\text{x}})|+\log(\text{k})$

$\Rightarrow\text{y}=\log\left\{\text{k}(1+\text{y})(1+\text{e}^{\text{x}})\right\}$

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