The Quadrature of the Parabola

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A parabolic segment.


The Quadrature of the Parabola is a treatise on geometry, written by Archimedes in the 3rd century B.C. Written as a letter to his friend Dositheus, the work presents 24 propositions regarding parabolas, culminating in a proof that the area of a parabolic segment (the region enclosed by a parabola and a line) is 4/3 that of a certain inscribed triangle.

The proof uses the method of exhaustion. Archimedes dissects the area into infinitely many triangles whose areas form a geometric progression. He computes the sum of the resulting geometric series, and proves that this is the area of the segment. This represents the most sophisticated use of the method of exhaustion in ancient mathematics, and remained unsurpassed until the development of integral calculus in the 17th century.

Main theorem

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Archimedes inscribes a certain triangle into the given parabolic segment.
A parabolic segment is the region bounded by a parabola and line. To find the area of a parabolic segment, Archimedes considers a certain inscribed triangle. The base of this triangle is the given chord of the parabola, and the third vertex is chosen so that the three vertical lines (parallel to the axis of the parabola) are equally spaced. The theorem is that the area of the parabolic segment is 4/3 that of the inscribed triangle.

Structure of the text

Archimedes gives two proofs of the main theorem. The first uses abstract mechanics, with Archimedes arguing that the weight of the segment will balance the weight of the triangle when placed on an appropriate lever. The second, more famous proof uses pure geometry, specifically the method of exhaustion.

Of the twenty-four propositions, the first three are quoted without proof from Euclid's Elements of Conics (a lost work by Euclid on conic sections). Propositions four and five establish elementary properties of the parabola; propositions six through seventeen give the mechanical proof of the main theorem; and propositions eighteen through twenty-four present the geometric proof.

Geometric proof

Dissection of the parabolic segment

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Archimedes' dissection of a parabolic segment into infinitely many triangles.
The main idea of the proof is the dissection of the parabolic segment into infinitely many triangles, as shown in the figure to the right. Each of these triangles in inscribed in its own parabolic segment in the same way that the blue triangle is inscribed in the large segment.

Areas of the triangles

In propositions eighteen through twenty-one, Archimedes proves that the area of each green triangle is one eighth of the area of the blue triangle. From a modern point of view, this is because the green triangle has half the width and a fourth of the height[1]:



By extension, each of the yellow triangles has one eighth the area of a green triangle, each of the red triangles has one eighth the area of a yellow triangle, and so on. Using the method of exhaustion, it follows that the total area of the parabolic segment is given by



Here T represents the area of the large blue triangle, the second term represents the total area of the two green triangles, the third term represents the total area of the four yellow triangles, and so forth. This simplifies to give

Sum of the series

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Archimedes' proof that 1/4 + 1/16 + 1/64 + ... = 1/3.
To complete the proof, Archimedes shows that



The expression on the left is a geometric series—each successive term is one fourth of the previous term. In modern mathematics, the formula above is a special case of the sum formula for a geometric series.

Archimedes evaluates the sum using an entirely geometric method[2], illustrated in the picture to the right. This picture shows a unit square which has been dissected into an infinity of smaller squares. Each successive purple square has one fourth the area of the previous square, with the total purple area being the sum



However, the purple squares are congruent to either set of yellow squares, and so cover 1/3 of the area of the unit square. It follows that the series above sums to 1/3.

Notes

1. ^ The green triangle has half of the width of blue triangle by construction. The statement about the height follows from the geometric properties of a parabola, and is easy to prove using modern analytic geometry.
2. ^ Strictly speaking, Archimedes evaluates the partial sums of this series, and uses the Archimedean property to argue that the partial sums become arbitrarily close to 4/3. This is logically equivalent to the modern idea of summing an infinite series.

See also

References

  • Ajose, Sunday and Roger Nelsen (June 1994). "Proof without Words: Geometric Series". Mathematics Magazine 67 (3): 230. 
  • id="CITEREFBressoud2006">Bressoud, David M. (2006), A Radical Approach to Real Analysis (2nd ed.), Mathematical Association of America, ISBN 0883857472.
    • id="CITEREFEdwards Jr.1994">Edwards Jr., C. H. (1994), The Historical Development of the Calculus, Springer, ISBN 0387943137.
      • Heath, Thomas L. (2005). The Works of Archimedes. Adamant Media Corporation. ISBN 1402171314. 
      • id="CITEREFSimmons2007">Simmons, George F. (2007), Calculus Gems, Mathematical Association of America, ISBN 0883855615.
        • Stein, Sherman K. (1999). Archimedes: What Did He Do Besides Cry Eureka?. Mathematical Association of America. ISBN 0883857189. 
        • id="CITEREFStillwell2004">Stillwell, John (2004), Mathematics and its History (2nd ed.), Springer, ISBN 0387953361.
          • Swain, Gordon and Thomas Dence (April 1998). "Archimedes' Quadrature of the Parabola Revisited". Mathematics Magazine 71 (2): 123–30. 
          • id="CITEREFWilson1995">Wilson, Alistair Macintosh (1995), The Infinite in the Finite, Oxford University Press, ISBN 0198539509.

            External Links

            Geometry (Greek γεωμετρία; geo = earth, metria = measure) is a part of mathematics concerned with questions of size, shape, and relative position of figures and with properties of space. Geometry is one of the oldest sciences.
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            Archimedes of Syracuse (Greek: Άρχιμήδης c. 287 BC – c. 212 BC) was an ancient Greek mathematician, physicist and engineer.
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            parabola (from the Greek: παραβολή) (IPA pronunciation: /pəˈrab(ə)lə/
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            line can be described as an ideal zero-width, infinitely long, perfectly straight curve (the term curve in mathematics includes "straight curves") containing an infinite number of points. In Euclidean geometry, exactly one line can be found that passes through any two points.
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            inscribed planar shape or solid is one that is enclosed by and "fits snugly" inside another geometric shape or solid. Specifically, there must be no object similar to the inscribed object but larger and also enclosed by the outer figure.
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            method of exhaustion is a method of finding the area of a shape by inscribing inside it a sequence of polygons whose areas converge to the area of the containing shape. If the sequence is correctly constructed, the difference in area between the n
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            A triangle is one of the basic shapes of geometry: a polygon with three corners or and three sides or edges which are straight line segments.

            In Euclidean geometry any three non-collinear points determine a triangle and a unique plane, i.e.
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            geometric progression, also known as a geometric sequence, is a sequence of numbers where each term after the first is found by multiplying the previous one by a fixed non-zero number called the common ratio. For example, the sequence 2, 6, 18, 54, ...
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            geometric series is a series with a constant ratio between successive terms. For example, the series



            is geometric, because each term is equal to half of the previous term.
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            INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.
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            A chord of a curve is a geometric line segment whose endpoints both lie on the curve. A secant or a secant line is the line extension of a chord.

            Chords of a circle

            Further information: Chord properties

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            Mechanics (Greek Μηχανική
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            lever (from French lever, "to raise", c.f. a levant) is a rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object.
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            method of exhaustion is a method of finding the area of a shape by inscribing inside it a sequence of polygons whose areas converge to the area of the containing shape. If the sequence is correctly constructed, the difference in area between the n
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            Euclid

            Born fl. 300 BC

            Residence Alexandria, Egypt
            Nationality Greek
            Field Mathematics
            Known for Euclid's Elements Euclid (Greek:
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            conic section (or just conic) is a curve that can be formed by intersecting a cone (more precisely, a right circular conical surface) with a plane. The conic sections were named and studied as long ago as 200 BC, when Apollonius of Perga undertook a systematic study of their
            ..... Click the link for more information.
            method of exhaustion is a method of finding the area of a shape by inscribing inside it a sequence of polygons whose areas converge to the area of the containing shape. If the sequence is correctly constructed, the difference in area between the n
            ..... Click the link for more information.
            geometric series is a series with a constant ratio between successive terms. For example, the series



            is geometric, because each term is equal to half of the previous term.
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            Analytic geometry, also called coordinate geometry and earlier referred to as Cartesian geometry or analytical geometry, is the study of geometry using the principles of algebra.
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            In mathematics, a series is often represented as the sum of a sequence of terms. That is, a series is represented as a list of numbers with addition operations between them, for example this arithmetic sequence:

            1 + 2 + 3 + 4 + 5 + ... + 99 + 100.

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            In abstract algebra, the Archimedean property, named after the ancient Greek mathematician Archimedes of Syracuse, is a property held by some groups, fields, and other algebraic structures.
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            Archimedes of Syracuse (Greek: Άρχιμήδης c. 287 BC – c. 212 BC) was an ancient Greek mathematician, physicist and engineer.
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            68 (3), p. 163-174.
            3. ^ Boyer, Carl B. (1991). "Archimedes of Syracuse", A History of Mathematics, Second Edition, John Wiley & Sons, Inc., 127. ISBN 0471543977.
            ..... Click the link for more information.
            method of exhaustion is a method of finding the area of a shape by inscribing inside it a sequence of polygons whose areas converge to the area of the containing shape. If the sequence is correctly constructed, the difference in area between the n
            ..... Click the link for more information.
            geometric series is a series with a constant ratio between successive terms. For example, the series



            is geometric, because each term is equal to half of the previous term.
            ..... Click the link for more information.


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