If $\sqrt{\frac{x}{y}}+\sqrt{\frac{y}{x}}=4$, then $\frac{d y}{d x}=$

If $\sqrt{\frac{x}{y}}+\sqrt{\frac{y}{x}}=4$, then $\frac{d y}{d x}=$
  1. $\frac{y-7 x}{7 x-y}$
  2. $\frac{7 y- x}{y-7 x}$
  3. $\frac{7 x+y}{x-7 y}$
  4. $\frac{y+7 x}{7y-x}$

Solution

Given : $\sqrt{\frac{x}{y}}+\sqrt{\frac{y}{x}}=4$ $\therefore \frac{x+y}{\sqrt{x y}}=4 \Rightarrow x+y=4 \sqrt{x y}$ Squaring both sides, we get, $(x+y)^{2}=16 x y \Rightarrow x^{2}+y^{2}=14 x y$ Differentiating both sides w.r.t. $x$ $\begin{array}{l} 2 x+2 y \frac{d y}{d x}=14\left[x \frac{d y}{d x}+y\right] \Rightarrow x+y \frac{d y}{d x}=7 x \cdot \frac{d y}{d x}+7 y \\ (y-7 x) \frac{d y}{d x}=7 y-x \Rightarrow \frac{d y}{d x}=\frac{7 y-x}{y-7 x} \end{array}$

Asked in: MHT CET 2020 (16 Oct Shift 1)

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