If $x^k+y^k=a^k(a, k>0)$ and $\frac{d y}{d x}+\left(\frac{y}{x}\right)^{\frac{1}{3}}-0$, then $\mathrm{k}$…

If $x^k+y^k=a^k(a, k>0)$ and $\frac{d y}{d x}+\left(\frac{y}{x}\right)^{\frac{1}{3}}-0$, then $\mathrm{k}$ has the value
  1. $\frac{1}{3}$
  2. $\frac{2}{3}$
  3. $\frac{1}{4}$
  4. $\frac{2}{7}$

Solution

$x^{\mathrm{k}}+y^{\mathrm{k}}=\mathrm{a}^{\mathrm{k}}$ Differentiating w.r.t. $x$, we get $\begin{aligned} & \mathrm{k} x^{k-1}+\mathrm{k} y^{\mathrm{k}-1} \frac{\mathrm{d} y}{\mathrm{~d} x}=0 \\ \therefore \quad & \frac{\mathrm{d} y}{\mathrm{~d} x}=-\frac{\mathrm{k} x^{\mathrm{k}-1}}{\mathrm{k} y^{\mathrm{k}-1}} \\ \therefore \quad & \frac{\mathrm{d} y}{\mathrm{~d} x}=-\left(\frac{x}{y}\right)^{\mathrm{k}-1} \\ & \frac{\mathrm{d} y}{\mathrm{~d} x}+\left(\frac{y}{x}\right)^{1-\mathrm{k}}=0 \\ & \text { But } \frac{\mathrm{d} y}{\mathrm{~d} x}+\left(\frac{y}{x}\right)^{\frac{1}{3}}=0 ...[Given] \end{aligned}$ Comparing above equations, we get $\begin{aligned} 1-k & =\frac{1}{3} \\ & 1-\frac{1}{3}=k \\ \therefore \quad k & =\frac{2}{3} \end{aligned}$

Asked in: MHT CET 2023 (09 May Shift 2)

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