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Zhao Youqin's π algorithm
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Zhao Youqin's π algorithm : ウィキペディア英語版
Zhao Youqin's π algorithm

Zhao Youqin's algorithm was an algorithm by Yuan dynasty astronomer-mathematician Zhao Youqin (, ? – 1330) to calculate the value of in his book ''Ge Xiang Xin Shu'' ().
== Algorithm ==
Zhao Youqin started with an inscribed square in a circle with radius r.〔Yoshio Mikami, Development of Mathematics in China and Japan, Chapter 20, The Studies about the Value of etc., pp 135–138〕
If \ell denotes the length of a side of the square, draw a perpendicular line d from the center of the circle to side l. Let e denotes r − d. Then from the diagram:
:d=\sqrt\right)^2}
:e=r-d=r-\sqrt\right)^2}.
Extend the perpendicular line d to dissect the circle into an octagon; \ell_2 denotes the length of one side of octagon.
:\ell_2=\sqrt\right)^2+e^2}
:\ell_2=\frac\sqrt \sqrt\right)^2}
let l_3 denotes the length of a side of hexadecagon
:\ell_3=\frac\sqrt\sqrt\right)^2 }
similarly
:\ell_=\frac\sqrt\sqrt\right)^2}
Proceeding in this way, he at last calculated the side of a 16384-gon, multiplying it by 16384 to obtain 3141.592 for a circle with diameter = 1000 units, or
:\pi =3.141592. \,
He multiplied this number by 113 and obtained 355. From this he deduced that of the traditional values of , that is 3, 3.14, and , the last is the most exact.〔Yoshio Mikami, p136〕

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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