Question
Choose the correct answer from the given four option.
Integrating factor of the differential equation $(1-\text{x}^2)\frac{\text{d}\text{y}}{\text{d}\text{x}}-\text{xy}=1$is:
  1. $-\text{x}$
  2. $\frac{\text{x}}{1+\text{x}^2}$
  3. $\sqrt{1-\text{x}^2}$
  4. $\frac{1}{2}\log(1-\text{x}^2)$

Answer

  1. $\sqrt{1-\text{x}^2}$
Solution:
Given is, $(1-\text{x}^2)\frac{\text{d}\text{y}}{\text{d}\text{x}}-\text{xy}=1$
$\Rightarrow\frac{\text{d}\text{y}}{\text{d}\text{x}}-\frac{\text{x}}{1-\text{x}^2}\text{y}=\frac{1}{1-\text{x}}^2$
Which is a linear differential equation.
$\therefore\text{I.F.}=\text{e}^{-\int\frac{\text{x}}{1-\text{x}^2}\text{dx}}$
Put $1-\text{x}^2=\text{t}$
$\Rightarrow-2\text{xdx}=\text{dt}$
$\Rightarrow\text{xdx}=-\frac{\text{dt}}{2}$
Now, $\text{I.F.}=\text{e}^{\frac{1}{2}\int\frac{\text{dt}}{\text{t}}}$
$\text{e}^{\frac{1}{2}\log\text{t}}=\text{e}^{\frac{1}{2}\log(1-\text{x}^2)}$
$\Rightarrow\sqrt{1-\text{x}^2}$

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