Minimising the perimeter of a tunnel with a fixed area

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SUMMARY

The problem involves minimizing the perimeter of a tunnel cross-section shaped as a rectangle topped with a semicircular roof, while maintaining a fixed area A. The perimeter is expressed as P = πr + 4r + 2h, where r is the radius of the semicircle and h is the height of the rectangle. The area constraint is given by A = 0.5πr² + 2rh. By establishing a relationship between r and h using the area equation, the problem can be simplified to a single variable optimization, which can be solved using differential calculus techniques.

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Homework Statement


A tunnel cross-section is to have the shape of a rectangle surmounted by a semicircular roof. The total cross-sectional area must be A, but the perimeter minimised to save building costs. Find its dimensions


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The Attempt at a Solution


I have that the perimeter would equal pi*r + 4r + 2h (where r is the radius of semi circle, h is height of rectangle) and that A=0.5pi*r^2 + 2rh, but am unsure as to how to get started. Any help would be appreciated.
 
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The fact that area must be equal to 'A' has given you a second equation. From it, you can determine the required relationship between r and h...meaning that the problem is reduced down to one variable (either r or h). Finding the value of that one variable that minimizes the perimeter is then just a differential calculus problem.
 

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