A rectangle of maximum area is inscribed in an ellipse $\frac{x^2}{25}+\frac{y^2}{16}=1$, then its…

A rectangle of maximum area is inscribed in an ellipse $\frac{x^2}{25}+\frac{y^2}{16}=1$, then its dimensions are
  1. $4 \sqrt{2}, 6 \sqrt{2}$
  2. $\sqrt{2}, 5 \sqrt{2}$
  3. $4 \sqrt{2}, 5 \sqrt{2}$
  4. $4 \sqrt{2}, \sqrt{2}$

Solution

Length of rectangle $=10 \cos \theta$ and breadth of rectangle $=8 \sin \theta$ $\therefore$ Area of rectangle $=(10 \cos \theta)(8 \sin \theta)=40(\sin \theta)$ Maximum area will occur when $\sin 2 \theta=1$ $\begin{aligned} & \therefore \sin 2 \theta=\sin \frac{\pi}{2} \quad \Rightarrow \theta=\frac{\pi}{4} \\ & \therefore P=\left(\frac{5}{\sqrt{2}}, \frac{4}{\sqrt{2}}\right) \Rightarrow \text { Dimensions of rectangle are } 5 \sqrt{2}, 4 \sqrt{2} \end{aligned}$

Asked in: MHT CET 2021 (22 Sep Shift 1)

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