Solve Word Problem: Find Rectangle Dimensions from Circular Piece of Sheet Metal

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Discussion Overview

The discussion revolves around solving a word problem involving a circular piece of sheet metal with a diameter of 20 inches, from which a rectangle of a specified area is to be formed. Participants explore the geometric and algebraic relationships between the circle and the rectangle, including the use of trigonometric expressions and algebraic equations.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states the problem and presents initial equations related to the circle and rectangle, including the area of the rectangle as 160 in².
  • Another participant interprets the problem and suggests using trigonometric expressions to find the rectangle's dimensions based on the circle's radius.
  • A different participant expresses uncertainty about using trigonometric functions, noting the lack of sufficient information about the triangle formed.
  • One participant provides a geometric approach, deriving equations involving the sides of the rectangle and the circle's diagonal, leading to a system of equations.
  • Another participant questions the multiplication of the area equation by 2, seeking clarification on the reasoning behind it.
  • A subsequent reply explains the algebraic manipulation of the equations, detailing how to derive expressions for the sum and difference of the rectangle's sides.
  • Another participant challenges the addition of values to the equations, asking for justification on when such manipulations are appropriate.
  • A later reply summarizes the algebraic approach, mentioning the use of the quadratic formula to solve for the rectangle's dimensions while noting the necessity to evaluate which roots are valid.

Areas of Agreement / Disagreement

Participants express various methods and approaches to solve the problem, with some agreeing on the algebraic manipulations while others question the validity of certain steps. No consensus is reached on a single method or solution.

Contextual Notes

There are unresolved assumptions regarding the application of trigonometric functions and the justification for algebraic manipulations. The discussion includes multiple approaches without a clear resolution on the correct method.

powp
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Hello All

I have this word problem that I am having problems with

Here it is

A circular piece of sheet metal has a diameter of 20 in. The edges are to be cut off to form a rectangle of area in^2. What are dimensions of the rectangle??

Here is what I have figured out. Not sure if it of any use

Formula of Circle is

10^2 = y ^ 2 + x ^ 2

Area of rectangle

160 = LW

Not sure how to go from here. Please help

Thanks
 
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powp said:
A circular piece of sheet metal has a diameter of 20 in. The edges are to be cut off to form a rectangle of area in^2. What are dimensions of the rectangle??

Here is what I have figured out. Not sure if it of any use

Formula of Circle is

10^2 = y ^ 2 + x ^ 2

Area of rectangle

160 = LW

Not sure how to go from here. Please help

Thanks

So, as I see it you have a circle of radius 10 and want to draw a rectangle in it with a total area of 160 right? Look at the plot below (drawn to scale) and where I have theta. Can you figure out what the length and height has to be using some trig expressions?
 

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Don't think I have enough info to use trig expessions. All I know for that triangle is the hypotenuse is 10 in and is a right angle triangle.

How can I determine the remaining info?
if I use sin I need opposite side?
 
IT’S ALL GEOMETRY…

X^2 + Y^2 = 20^2 = 400 ----------------------DIAGONAL OF THE RECTANGLE

XY = 160
2XY = 320

X^2 + 2XY + Y^2 = 400 + 320 = 720
(X+Y) ^2= 720


X+Y = 12 √5


X^2 – 2XY + Y^2 = 400 – 320 = 80
(X-Y)^ 2 = 80

X-Y = 4 √5

2 EQS., 2 UNKNOWNS

2X = 4 √5 + 12 √5 = 16 √5


X = 8 √5
Y = 4√5


 
Nice explanation plucker.
 
thanks why did you multiply xy = 160 by 2 ??
 
adding 2xy to x^2 + y^2 will yield to an expression x^2 + 2xy + y^2 which is the square of the sum of the sides of the rectangle...(x + y)

subtracting 2xy to x^2 + y^2 will yield to an expression x^2 - 2xy + y^2 which is the square of the difference of the sides of the rectangle...(x - y)

you will have 2 eqs and 2 unknows...
 
how can you just add values to the equation?? How do you know when to do this?
 
Hey Powp. Sorry for not get getting back with you last night but looks like these guys helped you ok. Yea, I didnt' see the obvious algegra:

x^2+y^2=400

xy=160

Solving for x or y in the second equation and substituting into the first leads to the same equation for both x and y:

r^4-400r^2+160^2=0

Now, using the quadratic formula, just figure out which of the 4 roots make sense.

I drew a pretty graph though. :wink:
 
Last edited:

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