What is the Optimal Rectangle in a Semicircle with Radius R?

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    Calculus Optimization
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Discussion Overview

The discussion revolves around finding the dimensions and area of the rectangle of largest area that can be inscribed in a semicircle of radius R, with one side of the rectangle lying on the diameter. The focus includes optimization techniques and mathematical reasoning related to this geometric problem.

Discussion Character

  • Mathematical reasoning
  • Homework-related
  • Exploratory

Main Points Raised

  • One participant expresses confusion about how to optimize the rectangle's area and seeks clarification on the requirements, including the need for a diagram.
  • Another participant provides a critical value for the rectangle's dimensions, specifically stating that the base of the rectangle is \(2x\) and asking for the height \(y\) at \(x=\frac{R}{\sqrt{2}}\).
  • A participant calculates the area \(A\) as \(2x \cdot \sqrt{R^2 - x^2}\) and seeks confirmation on whether they have correctly identified the length and width.
  • Further calculations are presented, with one participant attempting to express the area in terms of \(l\) and \(w\), leading to a more complex expression involving square roots.
  • Another participant confirms the base and height of the rectangle, ultimately calculating the area as \(R^2\) based on their derived dimensions.

Areas of Agreement / Disagreement

Participants present various calculations and approaches, but there is no consensus on the final area or the correctness of the derived expressions. The discussion remains unresolved regarding the optimal dimensions and area of the rectangle.

Contextual Notes

There are unresolved mathematical steps and assumptions regarding the optimization process, particularly in how the area is derived and expressed. The dependence on the critical value and the implications of the rectangle's dimensions are also not fully clarified.

drasord
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I'm really stuck on this problem. Could anyone provide some help?

Find the length and width of the rectangle of largest area that can be inscribed in a semicircle of radius R, assuming that one side of the rectangle lies on the diameter of the semicircle. Also, find the area of this rectangle. Draw a neat diagram.

Thanks!
 
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Can you show us what you have tried so far so we know where you are stuck and can best offer help?
 
Absolutely - sorry for the delay! This is my understanding of how to "optimize" the problem:

View attachment 1725

So I've found the area, I believe. But I need to find the "rectangle of largest area that can be inscribed in a semicircle of radius R". I'm confused about how to do this. And what does the professor mean by "a neat diagram"?
 

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You have found the correct critical value:

$$x=\frac{R}{\sqrt{2}}$$

The base of the rectangle is $2x$. The height is $y$.

So, what is $$y\left(\frac{R}{\sqrt{2}} \right)$$ ?
 

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Last edited:
Or now I have to find area:

A = l * w
A = 2R/sqrt(2) * sqrt(R^2/2)
A = sqrt(2) * sqrt(x) * sqrt(x^2)

?
 
The base of the rectangle is:

$$2x=2\cdot\frac{R}{\sqrt{2}}=\sqrt{2}R$$

The height is:

$$y=\sqrt{R^2-\frac{R^2}{2}}=\frac{R}{\sqrt{2}}$$

Thus area = base times height:

$$A=\sqrt{2}R\cdot\frac{R}{\sqrt{2}}=R^2$$
 

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