If $\mathrm{x}=\mathrm{e}^{\mathrm{t}}(\sin \mathrm{t}-\cos \mathrm{t})$ and…

If $\mathrm{x}=\mathrm{e}^{\mathrm{t}}(\sin \mathrm{t}-\cos \mathrm{t})$ and $\mathrm{y}=\mathrm{e}^{\mathrm{t}}(\sin \mathrm{t}+\cos \mathrm{t})$, then $\frac{\mathrm{dy}}{\mathrm{dx}}$ at $\mathrm{t}=\frac{\pi}{3}$ is
  1. $\sqrt{3}$
  2. $\frac{1}{\sqrt{3}}$
  3. $\frac{\sqrt{3}}{2}$
  4. $\frac{1}{2}$

Solution

$\begin{aligned} & x=e^t(\sin t-\cos t) \text { and } y=e^t(\sin t+\cos t) \\ & \therefore \quad \frac{d x}{d t}=e^t(\sin t-\cos t)+e^t(\cos t+\sin t)=2 e^t \sin t \\ & \frac{d y}{d t}=e^t(\sin t+\cos t)+e^t(\cos t-\sin t)=2 e^t \sin t \\ & \therefore \quad \frac{d y}{d x}=\frac{2 e^t \cos t}{2 e^t \sin t} \Rightarrow\left(\frac{d y}{d x}\right)_{t=\frac{\pi}{3}}=\cot \left(\frac{\pi}{3}\right)=\frac{1}{\sqrt{3}} \\ & \end{aligned}$

Asked in: MHT CET 2021 (24 Sep Shift 2)

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