# Ptolemy's theorem

Ptolemy's theorem is a relation among these lengths in a cyclic quadrilateral.

In Euclidean geometry, Ptolemy's theorem is a relation between the four sides and two diagonals of a cyclic quadrilateral (a quadrilateral whose vertices lie on a common circle). The theorem is named after the Greek astronomer and mathematician Ptolemy (Claudius Ptolemaeus).[1] Ptolemy used the theorem as an aid to creating his table of chords, a trigonometric table that he applied to astronomy.

If the quadrilateral is given with its four vertices A, B, C, and D in order, then the theorem states that:

${\displaystyle |{\overline {AC}}|\cdot |{\overline {BD}}|=|{\overline {AB}}|\cdot |{\overline {CD}}|+|{\overline {BC}}|\cdot |{\overline {AD}}|}$

where the vertical lines denote the lengths of the line segments between the named vertices. In the context of geometry, the above equality is often simply written as

This relation may be verbally expressed as follows:

If a quadrilateral is inscribable in a circle then the product of the measures of its diagonals is equal to the sum of the products of the measures of the pairs of opposite sides.

Moreover, the converse of Ptolemy's theorem is also true:

In a quadrilateral, if the sum of the products of its two pairs of opposite sides is equal to the product of its diagonals, then the quadrilateral can be inscribed in a circle.

## Examples

### Equilateral triangle

Equilateral triangle

Ptolemy's Theorem yields as a corollary a pretty theorem[2] regarding an equilateral triangle inscribed in a circle.

Given An equilateral triangle inscribed on a circle and a point on the circle.

The distance from the point to the most distant vertex of the triangle is the sum of the distances from the point to the two nearer vertices.

Proof: Follows immediately from Ptolemy's theorem:

${\displaystyle qs=ps+rs\Rightarrow q=p+r.}$

### Square

Any square can be inscribed in a circle whose center is the center of the square. If the common length of its four sides is equal to ${\displaystyle a}$ then the length of the diagonal is equal to ${\displaystyle a{\sqrt {2}}}$ according to the Pythagorean theorem and the relation obviously holds.

### Rectangle

Pythagoras' theorem: "manifestum est": Copernicus

More generally, if the quadrilateral is a rectangle with sides a and b and diagonal d then Ptolemy's theorem reduces to the Pythagorean theorem. In this case the center of the circle coincides with the point of intersection of the diagonals. The product of the diagonals is then d2, the right hand side of Ptolemy's relation is the sum a2 + b2.

Copernicus − who used Ptolemy's theorem extensively in his trigonometrical work − refers to this result as a 'Porism' or self-evident corollary:

Furthermore it is clear (manifestum est) that when the chord subtending an arc has been given, that chord too can be found which subtends the rest of the semicircle.[3]

### Pentagon

The golden ratio follows from this application of Ptolemy's theorem

A more interesting example is the relation between the length a of the side and the (common) length b of the 5 chords in a regular pentagon. In this case the relation reads b2 = a2 + ab which yields the golden ratio

${\displaystyle \varphi ={b \over a}={{1+{\sqrt {5}}} \over 2}.}$ [4]

### Side of decagon

Side of the inscribed decagon

If now diameter AF is drawn bisecting DC so that DF and CF are sides c of an inscribed decagon, Ptolemy's Theorem can again be applied – this time to cyclic quadrilateral ADFC with diameter d as one of its diagonals:

${\displaystyle ad=2bc}$
${\displaystyle \Rightarrow ad=2\varphi ac}$ where ${\displaystyle \varphi }$ is the golden ratio.
${\displaystyle \Rightarrow c={\frac {d}{2\varphi }}.}$[5]

whence the side of the inscribed decagon is obtained in terms of the circle diameter. Pythagoras' Theorem applied to right triangle AFD then yields "b" in terms of the diameter and "a" the side of the pentagon [6] is thereafter calculated as

${\displaystyle a={\frac {b}{\varphi }}=b(\varphi -1).}$

As Copernicus (following Ptolemy) wrote,

"The diameter of a circle being given, the sides of the triangle, tetragon, pentagon, hexagon and decagon, which the same circle circumscribes, are also given."[7]

## Proofs

### Proof by similarity of triangles

Constructions for a proof of Ptolemy's theorem

Let ABCD be a cyclic quadrilateral. On the chord BC, the inscribed angles ∠BAC = ∠BDC, and on AB, ∠ADB = ∠ACB. Construct K on AC such that ∠ABK = ∠CBD; since ∠ABK + ∠CBK = ∠ABC = ∠CBD + ∠ABD, ∠CBK = ∠ABD.

Now, by common angles △ABK is similar to △DBC, and likewise △ABD is similar △KBC. Thus AK/AB = CD/BD, and CK/BC = DA/BD; equivalently, AK·BD = AB·CD, and CK·BD = BC·DA. By adding two equalities we have AK·BD + CK·BD = AB·CD + BC·DA, and factorizing this gives (AK+CK)·BD = AB·CD + BC·DA. But AK+CK = AC, so AC·BD = AB·CD + BC·DA, Q.E.D.[8]

The proof as written is only valid for simple cyclic quadrilaterals. If the quadrilateral is self-crossing then K will be located outside the line segment AC. But in this case, AK−CK=±AC, giving the expected result.

### Proof by trigonometric identities

Let the inscribed angles subtended by ${\displaystyle AB}$, ${\displaystyle BC}$ and ${\displaystyle CD}$ be, respectively, ${\displaystyle \alpha }$, ${\displaystyle \beta }$ and ${\displaystyle \gamma }$, and the radius of the circle be ${\displaystyle R}$, then we have ${\displaystyle AB=2R\sin \alpha }$, ${\displaystyle BC=2R\sin \beta }$, ${\displaystyle CD=2R\sin \gamma }$, ${\displaystyle AD=2R\sin(\alpha +\beta +\gamma )}$, ${\displaystyle AC=2R\sin(\alpha +\beta )}$ and ${\displaystyle BD=2R\sin(\beta +\gamma )}$, and the original equality to be proved is transformed to

${\displaystyle \sin(\alpha +\beta )\sin(\beta +\gamma )=\sin \alpha \sin \gamma +\sin \beta \sin(\alpha +\beta +\gamma )}$

from which the factor ${\displaystyle 4R^{2}}$ has disappeared by dividing both sides of the equation by it.

Now by using the sum formulae, ${\displaystyle \sin(x+y)=\sin {x}\cos y+\cos x\sin y}$ and ${\displaystyle \cos(x+y)=\cos x\cos y-\sin x\sin y}$, it is trivial to show that both sides of the above equation are equal to

{\displaystyle {\begin{aligned}&\sin \alpha \sin \beta \cos \beta \cos \gamma +\sin \alpha \cos ^{2}\beta \sin \gamma \\+{}&\cos \alpha \sin ^{2}\beta \cos \gamma +\cos \alpha \sin \beta \cos \beta \sin \gamma .\end{aligned}}}

Q.E.D.

## Corollaries

${\displaystyle |S_{1}|=\sin(\theta _{1})}$
Corollary 1: Pythagoras' Theorem

In the case of a circle of unit diameter the sides ${\displaystyle S_{1},S_{2},S_{3},S_{4}}$ of any cyclic quadrilateral ABCD are numerically equal to the sines of the angles ${\displaystyle \theta _{1},\theta _{2},\theta _{3}}$ and ${\displaystyle \theta _{4}}$ which they subtend. Similarly the diagonals are equal to the sine of the sum of whichever pair of angles they subtend. We may then write Ptolemy's Theorem in the following trigonometric form:

${\displaystyle \sin \theta _{1}\sin \theta _{3}+\sin \theta _{2}\sin \theta _{4}=\sin(\theta _{3}+\theta _{2})\sin(\theta _{1}+\theta _{2})}$

Applying certain conditions to the subtended angles ${\displaystyle \theta _{1},\theta _{2},\theta _{3}}$ and ${\displaystyle \theta _{4}}$ it is possible to derive a number of important corollaries using the above as our starting point. In what follows it is important to bear in mind that the sum of angles ${\displaystyle \theta _{1}+\theta _{2}+\theta _{3}+\theta _{4}=180^{\circ }}$.

### Corollary 1. Pythagoras' theorem

Let ${\displaystyle \theta _{1}=\theta _{3}}$ and ${\displaystyle \theta _{2}=\theta _{4}}$. Then ${\displaystyle \theta _{1}+\theta _{2}=\theta _{3}+\theta _{4}=90^{\circ }}$ (since opposite angles of a cyclic quadrilateral are supplementary). Then:[9]

${\displaystyle \sin \theta _{1}\sin \theta _{3}+\sin \theta _{2}\sin \theta _{4}=\sin(\theta _{3}+\theta _{2})\sin(\theta _{3}+\theta _{4})}$
${\displaystyle \sin ^{2}\theta _{1}+\sin ^{2}\theta _{2}=\sin ^{2}(\theta _{1}+\theta _{2})}$
${\displaystyle \sin ^{2}\theta _{1}+\cos ^{2}\theta _{1}=1}$

### Corollary 2. The law of cosines

Corollary 2: The Law of Cosines

Let ${\displaystyle \theta _{2}=\theta _{4}}$. The rectangle of corollary 1 is now a symmetrical trapezium with equal diagonals and a pair of equal sides. The parallel sides differ in length by ${\displaystyle 2x}$ units where:

${\displaystyle x=S_{2}\cos(\theta _{2}+\theta _{3})}$

It will be easier in this case to revert to the standard statement of Ptolemy's theorem:

${\displaystyle {\begin{array}{lcl}\\S_{1}S_{3}+S_{2}S_{4}={\overline {AC}}\cdot {\overline {BD}}\\\Rightarrow S_{1}S_{3}+{S_{2}}^{2}={\overline {AC}}^{2}\\\Rightarrow S_{1}[S_{1}-2{S_{2}}\cos(\theta _{2}+\theta _{3})]+{S_{2}}^{2}={\overline {AC}}^{2}\\\Rightarrow {S_{1}}^{2}+{S_{2}}^{2}-2{S_{1}}{S_{2}}\cos(\theta _{2}+\theta _{3})={\overline {AC}}^{2}\\\end{array}}}$

The cosine rule for triangle ABC.

### Corollary 3: Compound angle sine (+)

Let

${\displaystyle \theta _{1}+\theta _{2}=\theta _{3}+\theta _{4}=90^{\circ }.}$

Then

${\displaystyle \sin \theta _{1}\sin \theta _{3}+\sin \theta _{2}\sin \theta _{4}=\sin(\theta _{3}+\theta _{2})\sin(\theta _{3}+\theta _{4})}$

Therefore,

${\displaystyle \cos \theta _{2}\sin \theta _{3}+\sin \theta _{2}\cos \theta _{3}=\sin(\theta _{3}+\theta _{2})\times 1}$

Formula for compound angle sine (+).[10]

### Corollary 4: Compound angle sine (−)

Let ${\displaystyle \theta _{1}=90^{\circ }}$. Then ${\displaystyle \theta _{2}+(\theta _{3}+\theta _{4})=90^{\circ }}$. Hence,

${\displaystyle \sin \theta _{1}\sin \theta _{3}+\sin \theta _{2}\sin \theta _{4}=\sin(\theta _{3}+\theta _{2})\sin(\theta _{3}+\theta _{4})}$
${\displaystyle \sin \theta _{3}+\sin \theta _{2}\cos(\theta _{2}+\theta _{3})=\sin(\theta _{3}+\theta _{2})\cos \theta _{2}}$
${\displaystyle \sin \theta _{3}=\sin(\theta _{3}+\theta _{2})\cos \theta _{2}-\cos(\theta _{2}+\theta _{3})\sin \theta _{2}}$

Formula for compound angle sine (−).[10]

This derivation corresponds to the Third Theorem as chronicled by Copernicus following Ptolemy in Almagest. In particular if the sides of a pentagon (subtending 36° at the circumference) and of a hexagon (subtending 30° at the circumference) are given, a chord subtending 6° may be calculated. This was a critical step in the ancient method of calculating tables of chords.[11]

### Corollary 5: Compound angle cosine (+)

This corollary is the core of the Fifth Theorem as chronicled by Copernicus following Ptolemy in Almagest.

Let ${\displaystyle \theta _{3}=90^{\circ }}$. Then ${\displaystyle \theta _{1}+(\theta _{2}+\theta _{4})=90^{\circ }}$. Hence

${\displaystyle \sin \theta _{1}\sin \theta _{3}+\sin \theta _{2}\sin \theta _{4}=\sin(\theta _{3}+\theta _{2})\sin(\theta _{3}+\theta _{4})}$
${\displaystyle \cos(\theta _{2}+\theta _{4})+\sin \theta _{2}\sin \theta _{4}=\cos \theta _{2}\cos \theta _{4}}$
${\displaystyle \cos(\theta _{2}+\theta _{4})=\cos \theta _{2}\cos \theta _{4}-\sin \theta _{2}\sin \theta _{4}}$

Formula for compound angle cosine (+)

Despite lacking the dexterity of our modern trigonometric notation, it should be clear from the above corollaries that in Ptolemy's theorem (or more simply the Second Theorem) the ancient world had at its disposal an extremely flexible and powerful trigonometric tool which enabled the cognoscenti of those times to draw up accurate tables of chords (corresponding to tables of sines) and to use these in their attempts to understand and map the cosmos as they saw it. Since tables of chords were drawn up by Hipparchus three centuries before Ptolemy, we must assume he knew of the 'Second Theorem' and its derivatives. Following the trail of ancient astronomers, history records the star catalogue of Timocharis of Alexandria. If, as seems likely, the compilation of such catalogues required an understanding of the 'Second Theorem' then the true origins of the latter disappear thereafter into the mists of antiquity but it cannot be unreasonable to presume that the astronomers, architects and construction engineers of ancient Egypt may have had some knowledge of it.

## Ptolemy's inequality

This is not a cyclic quadrilateral. The equality never holds here, and is unequal in the direction indicated by Ptolemy's inequality.

The equation in Ptolemy's theorem is never true with non-cyclic quadrilaterals. Ptolemy's inequality is an extension of this fact, and it is a more general form of Ptolemy's theorem. It states that, given a quadrilateral ABCD, then

${\displaystyle {\overline {AB}}\cdot {\overline {CD}}+{\overline {BC}}\cdot {\overline {DA}}\geq {\overline {AC}}\cdot {\overline {BD}}}$

where equality holds if and only if the quadrilateral is cyclic. This special case is equivalent to Ptolemy's theorem.

## Notes

1. ^ C. Ptolemy, Almagest, Book 1, Chapter 10.
2. ^ Wilson, Jim. "Ptolemy's Theorem." link verified 2009-04-08
3. ^ De Revolutionibus Orbium Coelestium: Page 37. See last two lines of this page. Copernicus refers to Ptolemy's theorem as "Theorema Secundum".
4. ^ Proposition 8 in Book XIII of Euclid's Elements proves by similar triangles the same result: namely that length a (the side of the pentagon) divides length b (joining alternate vertices of the pentagon) in "mean and extreme ratio".
5. ^ And in analogous fashion Proposition 9 in Book XIII of Euclid's Elements proves by similar triangles that length c (the side of the decagon) divides the radius in "mean and extreme ratio".
6. ^ An interesting article on the construction of a regular pentagon and determination of side length can be found at the following reference [1]
7. ^ De Revolutionibus Orbium Coelestium: Liber Primus: Theorema Primum
8. ^ Alsina, Claudi; Nelsen, Roger B. (2010), Charming Proofs: A Journey Into Elegant Mathematics, Dolciani Mathematical Expositions, 42, Mathematical Association of America, p. 112, ISBN 9780883853481.
9. ^ In De Revolutionibus Orbium Coelestium, Copernicus does not refer to Pythagoras' Theorem by name but uses the term 'Porism' – a word which in this particular context would appear to denote an observation on – or obvious consequence of – another existing theorem. The 'Porism' can be viewed on pages 36 and 37 of DROC (Harvard electronic copy)
10. ^ a b
11. ^ To understand the Third Theorem, compare the Copernican diagram shown on page 39 of the Harvard copy of De Revolutionibus to that for the derivation of sin(A-B) found in the above cut-the-knot web page

## References

• Coxeter, H. S. M. and Greitzer, S. L.: "Ptolemy's Theorem and its Extensions." §2.6 in Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 42–43, 1967.
• De Revolutionibus Orbium Coelestium, Copernicus, Nicolaus. English translation from On the Shoulders of Giants, Hawking, S 2002, Penguin Books. ISBN 0-14-101571-3
• Amarasinghe, G.W.I.S. A Concise Elementary Proof for the Ptolemy's Theorem, Global Journal of Advanced Research on Classical and Modern Geometries(GJARCMG), Vol 02(01), pp. 20–25, 2013.