Math 409 409g history of mathematics
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Math 409/409G History of Mathematics. Quadrature of Polygons. What is quadrature?. The quadrature (or squaring) of a plane figure is the construction – using only straightedge and compass – of a square having area equal to that of the original plane figure.

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Math 409/409G History of Mathematics

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Math 409 409g history of mathematics

Math 409/409GHistory of Mathematics

Quadrature of Polygons


What is quadrature

What is quadrature?

The quadrature (or squaring) of a plane figure is the construction – using only straightedge and compass – of a square having area equal to that of the original plane figure.


Math 409 409g history of mathematics

The quadrature problem appealed to the ancient Greeks because finding the area of an irregularly shaped figure is not easy.

Being able to replace the irregular figure with an equivalent square, would make determining its area a trivial matter.


Squaring a rectangle

Squaring a rectangle

  • Start with a rectangle ABCD with sides measuring x and y, as pictured.

  • Construct point E on side AD extended such that DE measures x.


Math 409 409g history of mathematics

  • Construct the midpoint F of segment AE.

  • Denote the measure of AF by a and the measure of FD by b.

    Then

    and


Math 409 409g history of mathematics

  • Construct a circle centered at F and having radius a.

  • Let G denote the point where this circle intersects the extension of side DC, as pictured.

  • Denote the measure of GD by c.


Math 409 409g history of mathematics

Since FG is a radius of the circle, it has measure a.

So by the Pythagorean theorem,

And thus,


Math 409 409g history of mathematics

  • Construct a square having side GD, as pictured.


Math 409 409g history of mathematics

Then:

This shows that the ancient Greeks were able to square a rectangle.


Squaring a triangle

Squaring a triangle

Given a triangle having base measuring b and height measuring h, construct a rectangle having sides measuring h/2 and b.


Math 409 409g history of mathematics

Then square the rectangle.

The area of the triangle is then equal to the area of the square.


Squaring a polygon

Squaring a polygon

By drawing diagonals, a polygon can be subdivided into triangular regions. And each of these triangles can be squared.


Math 409 409g history of mathematics

So squaring a polygon boils down to showing that the sum of two squares can be squared.


Squaring the sum of two squares

Squaring the sum of two squares

Let the sides of the two squares measure a and b respectively.

Place these squares so they form a right angle, as pictured.

Let c denote the measure of the hypotenuse of the right triangle formed by the sides of the two squares.


Math 409 409g history of mathematics

By the Pythagorean theorem, a2+ b2 = c2.

So constructing a square having the hypotenuse of the triangle as a side results in a square whose area is equal to the sum of the two original squares.


Math 409 409g history of mathematics

Sometimes it’s easier to express the area of a polygon as the difference of squares

than as the sum of squares.


Squaring the difference of two squares

Squaring the difference of two squares

  • Let the two squares have sides measuring a and b, respectively, with a> b. Place these squares as pictured.

  • Construct a circle centered at P and having radius a.


Math 409 409g history of mathematics

  • Construct segment QR as pictured and denote it’s measure by c.

  • Construct segment PQ. Since PQ is a radius of the circle, its measure is a.


Math 409 409g history of mathematics

  • Construct a square having side QR, as pictured.

    Since PQR is a right triangle, a2 – b2 = c2.

    So


Summary

Summary

We have seen how to:

  • Square a rectangle.

  • Square a triangle.

  • Square the sum of two squares.

  • Square the difference of two squares.

    And we saw that you could square a polygonal region by subdividing it into triangles, squaring the triangles, and the combining the “squared triangles” into a single square by adding and/or subtracting them.


Math 409 409g history of mathematics

This ends the lesson on

Quadrature of Polygons


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