圓者,繞心一周而距心俱等者也。《墨子》曰:「一中同長」,是之謂也。亦圓錐曲線耳。
凡直接周心者,曰半徑,其長咸同耳。
至於圓之積者,半周、半徑相乘得積步。
周上二點相接,曰弦;其貫心者,曰徑;周徑之比,曰圓周率。古云周三徑一。及至南朝,祖沖之始精其義,以百一十三分之三百五十五為密率,以七分之廿二為約率。
又,二半徑之交,曰心角。二弦同源,所交之角,曰周角,蓋為心角之半耳。
若夫弦之所切,二分其圓,而周之分也,曰弧;以半徑乘心角,可得其長。凡分圓以半徑再,可得扇形;欲知其積步,先取半徑之方,復乘心角,然後半之。
至若弦與弧之合,弓形也。
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