The differential equation of the family of curves,

Question:

The differential equation of the family of curves,

$x^{2}=4 b(y+b), b \in R$, is:

  1. (1) $x\left(y^{\prime}\right)^{2}=x+2 y y^{\prime}$

  2. (2) $x\left(y^{\prime}\right)^{2}=2 y y^{\prime}-x$

  3. (3) $x y^{\prime \prime}=y^{\prime}$

  4. (4) $x\left(y^{\prime}\right)^{2}=x-2 y y^{\prime}$


Correct Option: 1,

Solution:

Since, $x^{2}=4 b(y+b)$

$x^{2}=4 b y+4 b^{2}$

$2 x=4 b y^{\prime}$

$\Rightarrow \quad b=\frac{x}{2 y^{\prime}}$

So, differential equation is

$x^{2}=\frac{2 x}{y^{\prime}} \cdot y+\left(\frac{x}{y^{\prime}}\right)^{2}$

$x\left(y^{\prime}\right)^{2}=2 y y^{\prime}+x$

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