Show square inscribed in circle has maximum area

In summary, the conversation discusses how to prove that the square has the largest area among all 4-sided polygons inscribed in a circle. The suggested approach is to express the surface area of the polygon as a function of the length of its sides and find the maximum value. One method involves using coordinate geometry and the parametric equation of a circle, while another involves using the area formula for a quadrilateral. It is ultimately proven that the square has the maximum area when its sides are equal and the angles are 90 degrees.
  • #1
t_n_p
595
0

Homework Statement


Show that the square, when inscribed in a circle, has the largest area of all the 4-sided polygons. Try to show that all sides of a quadrilateral of maximal area have to be of
equal length.

The Attempt at a Solution


How do you start?
I don't get how showing the sides are equal will help prove that the square has maximal area.
Any hints?
 
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  • #2
Try to express surface of the inscribed 4-sided polygon as a function of the length of its sides. General case will be more difficult, but for a specific case of rectangle it should be relatively easy to express lengths as a function of one variable.

Once you have that, find maximum.
 
  • #3
Borek said:
Try to express surface of the inscribed 4-sided polygon as a function of the length of its sides. General case will be more difficult, but for a specific case of rectangle it should be relatively easy to express lengths as a function of one variable.

Once you have that, find maximum.

I don't quite understand.
Say I have a square, let the side length = a.

area = a^2.
Then maximize by taking derivative?
 
  • #4
Real easy to prove with coordinate geometry, but you need to know the parametric equation of a circle and the area of a quadrilateral.

Any point on a circle (with origin as centre and radius r) is (r cosx, r sinx)
and area of a quadrilateral with vertices (x1, y1), (x2, y2), (x3, y3), (x4, y4),
is #8 here http://www.mathisfunforum.com/viewtopic.php?id=3301

take four points, put them in the formula, the result becomes obvious
 
  • #5
that gives me the area, but it doesn't prove that it is the maximum area for a 4 sided shape.
 
  • #6
it does! the result for area of a general quadrilateral in a circle comes to be (assuming radius r and angles A, B, C, D
r2/2[sin(A-B) + sin(B-C) + sin(C-D) + sin(D-A)]
the maximum value of which (2r2) is attained only when each of the angular differences is 90 degrees which results in a square

did u try solving it? you need basic trignometric formulae
 

Related to Show square inscribed in circle has maximum area

1. How do you prove that a square inscribed in a circle has the maximum area?

The proof involves using the properties of a circle and a square to show that the area of the square is equal to the area of the circle. This can be done using geometrical constructions and mathematical equations.

2. What is the formula for finding the maximum area of a square inscribed in a circle?

The formula is A = (π/4) * r^2, where A is the area of the square and r is the radius of the circle. This formula is derived from the fact that the diagonal of the square is equal to the diameter of the circle, and the area of a square is calculated by multiplying the length of its side by itself.

3. Why does a square inscribed in a circle have the maximum area?

This is because a square is the only quadrilateral that can be inscribed in a circle and have all of its vertices touching the circumference of the circle. This results in the square having a larger area compared to any other quadrilateral inscribed in the same circle.

4. Can the maximum area of a square inscribed in a circle be calculated without using a formula?

Yes, it is possible to visually see that a square inscribed in a circle has the maximum area by using geometrical constructions and comparing the areas of different shapes inscribed in the same circle. However, using the formula will provide a more accurate and efficient method of calculation.

5. How is the concept of a maximum area of a square inscribed in a circle useful in real life?

This concept is useful in many areas such as architecture, engineering, and mathematics. It can be applied in designing structures with maximum stability and efficiency, optimizing the use of space in various applications, and solving various mathematical problems involving circles and squares.

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