Zhu Shijie - Jade Mirror of The Four Unknowns

Zhu's second book, Jade Mirror of the Four Unknowns (四元玉鉴, Siyuan yujian), written in 1303, is his most important work. With this book, Zhu brought Chinese algebra to its highest level. The first four of the 288 problems for solution illustrate his method of the four unknowns. He shows how to convert a problem stated verbally into a system of polynomial equations (up to 14th order), by using up to four unknowns: 天Heaven, 地Earth, 人Man, 物Matter,and then how to reduce the system to a single polynomial equation in one unknown by successive elimination of unknowns. He then solved the high order equation by Southern Song dynasty mathematician Qin Jiushao's "Ling long kai fang" method published in Shùshū Jiǔzhāng (“Mathematical Treatise in Nine Sections”) in 1247 (more than 570 years before English mathematician William Horner's method using synthetic division). To do this, he makes use of the Pascal triangle, which he labels as the diagram of an ancient method first discovered by Jia Xian before 1050. The final equation and one of its solutions is given for each of the 288 problems. Zhu also found square and cube roots by solving quadratic and cubic equations, and added to the understanding of series and progressions, classifying them according to the coefficients of the Pascal triangle. He also showed how to solve systems of linear equations be reducing the matrix of their coefficents to diagonal form. His methods pre-date Blaise Pascal, William Horner, and modern matrix methods by many centuries. The preface of the book describes how Zhu travelled around China for 20 years as a teacher of mathematics.