Ceyuan haijing (simplified Chinese: 测圆海镜; traditional Chinese: 測圓海鏡; pinyin: cè yuán hǎi jìng; lit. 'sea mirror of circle measurements') is a treatise on solving geometry problems with the algebra of Tian yuan shu written by the mathematician Li Zhi in 1248 in the time of the Mongol Empire. It is a collection of 692 formulas and 170 problems, all derived from the same master diagram of a "round town" inscribed in a right triangle and a square. They often involve two people who walk on straight lines until they can see each other, meet or reach a tree or pagoda in a certain spot.
The majority of the geometry problems are solved by polynomial equations, which are represented using a method called tian yuan shu, "coefficient array method" or literally "method of the celestial unknown". Li Zhi is the earliest extant source of this method, though it was known before him in some form. It is a positional system of rod numerals to represent polynomial equations.
Ceyuan haijing was first introduced to the west by the British Protestant Christian missionary to China, Alexander Wylie in his book Notes on Chinese Literature, 1902. He wrote:The first page has a diagram of a circle contained in a triangle, which is dissected into 15 figures; the definition and ratios of the several parts are then given, and there are followed by 170 problems, in which the principle of the new science are seen to advantage. There is an exposition and scholia throughout by the author.
This treatise consists of 12 volumes.
Contents
Volume 1
Diagram of a Round Town
The monography begins with a master diagram called the Diagram of Round Town(圆城图式). It shows a circle inscribed in a right angle triangle and four horizontal lines, four vertical lines.
TLQ, the large right angle triangle, with horizontal line LQ, vertical line TQ and hypotenuse TL
C: Center of circle:
NCS: A vertical line through C, intersect the circle and line LQ at N(南north side of city wall), intersects south side of circle at S(南).
NCSR, Extension of line NCS to intersect hypotenuse TL at R(日)
WCE: a horizontal line passing center C, intersects circle and line TQ at W(西, west side of city wall) and circle at E (东, east side of city wall).
WCEB:extension of line WCE to intersect hypotenuse at B(川)
KSYV: a horizontal tangent at S, intersects line TQ at K(坤), hypotenuse TL at Y(月).
HEMV: vertical tangent of circle at point E, intersects line LQ at H, hypotenuse at M(山, mountain)
HSYY, KSYV, HNQ, QSK form a square, with inscribed circle C.
Line YS, vertical line from Y intersects line LQ at S(泉, spring)
Line BJ, vertical line from point B, intersects line LQ at J(夕, night)
RD, a horizontal line from R, intersects line TQ at D(旦, day)
The North, South, East and West direction in Li Zhi's diagram are opposite to our present convention.
Triangles and their sides
There are a total of fifteen right angle triangles formed by the intersection between triangle TLQ, the four horizontal lines, and four vertical lines.
The names of these right angle triangles and their sides are summarized in the following table
In problems from Vol 2 to Vol 12, the names of these triangles are used in very terse terms. For instance
"明差","MING difference" refers to the "difference between the vertical side and horizontal side of MING triangle.
"叀差","ZHUANG difference" refers to the "difference between the vertical side and horizontal side of ZHUANG triangle."
"明差叀差并" means "the sum of MING difference and ZHUAN difference"
(
b
14
−
a
14
)
+
(
b
15
−
a
15
)
{\displaystyle (b_{14}-a_{14})+(b_{15}-a_{15})}
Length of Line Segments
This section (今问正数) lists the length of line segments, the sum and difference and their combinations in the diagram of round town, given that the radius r of inscribe circle is
r
=
120
{\displaystyle r=120}
paces
a
1
=
320
{\displaystyle a_{1}=320}
,
b
1
=
640
{\displaystyle b_{1}=640}
.
The 13 segments of ith triangle (i=1 to 15) are:
Hypoteneuse
c
i
{\displaystyle c_{i}}
Horizontal
a
i
{\displaystyle a_{i}}
Vertical
b
i
{\displaystyle b_{i}}
Definitions and formula
(
c
1
−
a
1
)
∗
(
c
1
∗
b
1
)
{\displaystyle (c_{1}-a_{1})*(c_{1}*b_{1})}
=
1
2
{\displaystyle 1 \over 2}
*
(
d
1
)
2
{\displaystyle (d_{1})^{2}}
a
10
∗
b
11
{\displaystyle a_{10}*b_{11}}
=
1
2
Volume 2
This volume begins with a general hypothesis
All subsequent 170 problems are about given several segments, or their sum or difference, to find the radius or diameter of the round town. All problems follow more or less the same format; it begins with a Question, followed by description of algorithm, occasionally followed by step by step description of the procedure.
Nine types of inscribed circle
The first ten problems were solved without the use of Tian yuan shu. These problems are related to
various types of inscribed circle.
Question 1
Two men A and B start from corner Q. A walks eastward 320 paces and stands still. B walks southward 600 paces and see B. What is the diameter of the circular city ?
Answer: the diameter of the round town is 240 paces.
This is inscribed circle problem associated with
△
T
L
Q
{\displaystyle \triangle TLQ}
Algorithm:
d
=
2
a
1
×
b
Tian yuan shu
From problem 14 onwards, Li Zhi introduced "Tian yuan one" as unknown variable, and set up two expressions according to Section Definition and formula, then equate these two tian yuan shu expressions. He then solved the problem and obtained the answer.
Question 14:"Suppose a man walking out from West gate and heading south for 480 paces and encountered a tree. He then walked out from the North gate heading east for 200 paces and saw the same tree. What is the radius of the round own?"
Algorithm: Set up the radius as Tian yuan one, place the counting rods representing southward 480 paces on the floor, subtract the tian yuan radius to obtain
：
480
−
x
{\displaystyle 480-x}
元
。
Then subtract tian yuan from eastward paces 200 to obtain:
200
−
x
{\displaystyle 200-x}
元
multiply these two expressions to get：
x
2
−
680
x
+
96000
{\displaystyle x^{2}-680x+96000}
Volume 3
17 problems associated with segment
b
2
{\displaystyle b_{2}}
i.e TW in
△
T
W
B
{\displaystyle \triangle TWB}
The
a
10
{\displaystyle a_{10}}
pairs with
b
11
{\displaystyle b_{11}}
,
a
11
{\displaystyle a_{11}}
pairs with
b
10
{\displaystyle b_{10}}
and
a
15
{\displaystyle a_{15}}
pairs with
b
14
Volume 4
17 problems, given
a
3
{\displaystyle a_{3}}
and a second segment, find diameter of circular city.
。
Volume 5
18 problems, given
b
1
{\displaystyle b_{1}}
。
Volume 6
18 problems.
Q1-11，13-19 given
a
1
{\displaystyle a_{1}}
，and a second line segment, find diameter d.
Q12：given
a
1
+
c
3
{\displaystyle a_{1}+c_{3}}
and another line segment, find diameter d.
Volume 7
18 problems, given two line segments find the diameter of round town
Volume 8
17 problems, given three to eight segments or their sum or difference, find diameter of round city.
Problem 14
Given the sum of GAO difference and MING difference is 161 paces and the sum of MING difference and ZHUAN difference is 77 paces. What is the diameter of the round city?
Answer: 120 paces.
Algorithm:
Given
(
b
7
−
a
7
)
+
(
b
8
−
a
8
)
=
161
{\displaystyle (b_{7}-a_{7})+(b_{8}-a_{8})=161}
(
b
14
−
a
14
)
+
(
b
15
−
Volume 9
Part I
Part II
Volume 10
8 problems
Volume 11
：Miscellaneous 18 problems：
Volume 12
14 problems on fractions
Research
In 1913, French mathematician L. van Hoe wrote an article about Ceyuan haijing. In 1982, K. Chemla Ph.D. thesis Etude du Livre Reflects des Mesuers du Cercle sur la mer de Li Ye. 1983, University of Singapore Mathematics Professor Lam Lay Yong: Chinese Polynomial Equations in the Thirteenth Century。