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an estimate of π in the comments to chapter 1. He estimated pi to 3.141014 with a 192 sided polygon and later calculated pi as 3.14159 by using a 3079 sided polygon. He suggested that 3.14 was a good approximation. His estimation is made with a method similar to Archimedes. The Nine Chapters used the value 3 as π, but Zhang Heng had previously estimated it to the square root of 10;

Gaussian elimination;

Cavalieri's principle to find the volume of a cylinder.