If $y=\sqrt{x+\sqrt{x+\sqrt{x+\ldots \ldots \infty}}}$, then $\frac{d y}{d x}$ is equal to

If $y=\sqrt{x+\sqrt{x+\sqrt{x+\ldots \ldots \infty}}}$, then $\frac{d y}{d x}$ is equal to
  1. $\frac{1}{y}$
  2. $\frac{1}{x}$
  3. $\frac{1}{2 x-1}$
  4. $\frac{1}{2 y-1}$

Solution

We have, $ \begin{aligned} y & =\sqrt{x+\sqrt{x+\sqrt{x+\ldots}}} \\ y^2 & =x+\sqrt{x+\sqrt{x+\sqrt{x+\sqrt{x+\ldots}}}} \\ y^2 & =x+y \\ y^2-y & =x \end{aligned} $ On differentiating w.r.t. $x$, we get $ \begin{aligned} (2 y-1) \frac{d y}{d x} & =1 \\ \frac{d y}{d x} & =\frac{1}{2 y-1} \end{aligned} $

Asked in: AP EAMCET 2021 (25 Aug Shift 1)

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