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# Solution for A Window in the Form of a Rectangle is Surmounted by a Semi-circular Opening. the Total Perimeter of the Window is 10 M. Find the Dimension of the Rectangular of the Window to Admit - CBSE (Commerce) Class 12 - Mathematics

#### Question

A window in the form of a rectangle is surmounted by a semi-circular opening. The total perimeter of the window is 10 m. Find the dimension of the rectangular of the window to admit maximum light through the whole opening.

#### Solution

$\text { Let the dimensions of the rectangular part be x and y }.$

$\text { Radius of semi-circle } =\frac{x}{2}$

$\text { Total perimeter } = 10$

$\Rightarrow \left( x + 2y \right) + \pi\left( \frac{x}{2} \right) = 10$

$\Rightarrow 2y = \left[ 10 - x - \pi\left( \frac{x}{2} \right) \right]$

$\Rightarrow y = \frac{1}{2}\left[ 10 - x\left( 1 + \frac{\pi}{2} \right) \right] . . . \left( 1 \right)$

$\text { Now },$

$\text { Area }, A = \frac{\pi}{2} \left( \frac{x}{2} \right)^2 + xy$

$\Rightarrow A = \frac{\pi x^2}{8} + \frac{x}{2}\left[ 10 - x\left( 1 + \frac{\pi}{2} \right) \right] \left[ \text { From eq } . \left( 1 \right) \right]$

$\Rightarrow A = \frac{\pi x^2}{8} + \frac{10x}{2} - \frac{x^2}{2}\left( 1 + \frac{\pi}{2} \right)$

$\Rightarrow \frac{dA}{dx} = \frac{\pi x}{4} + \frac{10}{2} - \frac{2x}{2}\left( 1 + \frac{\pi}{2} \right)$

$\text { For maximum or minimum values of A, we must have }$

$\frac{dA}{dx} = 0$

$\Rightarrow \frac{\pi x}{4} + \frac{10}{2} - \frac{2x}{2}\left( 1 + \frac{\pi}{2} \right) = 0$

$\Rightarrow x\left[ \frac{\pi}{4} - 1 - \frac{\pi}{2} \right] = - 5$

$\Rightarrow x = \frac{- 5}{\left( \frac{- 4 - \pi}{4} \right)}$

$\Rightarrow x = \frac{20}{\left( \pi + 4 \right)}$

$\text { Substituting the value ofxin eq }. \left( 1 \right), \text { we get }$

$y = \frac{1}{2}\left[ 10 - \left( \frac{20}{\pi + 4} \right)\left( 1 + \frac{\pi}{2} \right) \right]$

$\Rightarrow y = 5 - \frac{10\left( \pi + 2 \right)}{2\left( \pi + 4 \right)}$

$\Rightarrow y = \frac{5\pi + 20 - 5\pi - 10}{\left( \pi + 4 \right)}$

$\Rightarrow y = \frac{10}{\left( \pi + 4 \right)}$

$\frac{d^2 A}{d x^2} = \frac{\pi}{4} - \frac{\pi}{2} - 1$

$\Rightarrow \frac{d^2 A}{d x^2} = \frac{\pi - 2\pi - 4}{4}$

$\Rightarrow \frac{d^2 A}{d x^2} = \frac{- \pi - 4}{4} < 0$

$\text { Thus, the area is maximum when x= }\frac{20}{\pi + 4}\text { and} y=\frac{10}{\pi + 4}.$

$\text { So, the required dimensions are given below }:$

$\text { Length } = \frac{20}{\pi + 4} m$

$\text { Breadth }=\frac{10}{\pi + 4}m$

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Solution A Window in the Form of a Rectangle is Surmounted by a Semi-circular Opening. the Total Perimeter of the Window is 10 M. Find the Dimension of the Rectangular of the Window to Admit Concept: Graph of Maxima and Minima.
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