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