If $x^{2}+y^{2}=1$, then $\frac{d^{2} x}{d y^{2}}=$

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

Solution

We have $x^{2}+y^{2}=1$ $\begin{array}{l} 2 x+2 y \frac{d y}{d x}=0 \Rightarrow 2 y \frac{d y}{d x}=-2 x \\ \frac{d y}{d x}=-\frac{x}{y} \Rightarrow \frac{d x}{d y}=-\frac{y}{x} \end{array}$ Differentiating w.r.t. $\mathrm{y}$ we get $\frac{d^{2} x}{d y^{2}}=\left[\frac{x(1)-y \frac{d x}{d y}}{x^{2}}\right]=-\left[\frac{x-y\left(\frac{-y}{x}\right)}{x^{2}}\right]=-\left[\frac{x^{2}+y^{2}}{x^{3}}\right]$ $=-\frac{1}{x^{3}} \quad \cdots\left[\because x^{2}+y^{2}=1\right.$, given $]$

Asked in: MHT CET 2020 (13 Oct Shift 1)

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