elem.4.1
在给定圆内作一条线段，使其等于一条不大于圆直径的给定线段。
圆 ABC 圆心 O，BC 为直径；给定线段 D；在 BC 上取 CE=D，以 C 为圆心、CE 为半径作小圆 EAF 交大圆于 A，弦 CA 即等于 D。
正文图形由校订坐标生成；点、线、角、圆可与证明和问答联动。
Let ABC be the given circle, and D the given straight line not greater than the diameter of the circle; thus it is required to fit into the circle ABC a straight line equal to the straight line D. Let a diameter BC of the circle ABC be drawn.
设ABC为给定圆，D为不大于圆直径的给定线段。作直径BC。
Then, if BC is equal to D, that which was enjoined will have been done; for BC has been fitted into the circle ABC equal to the straight line D.
若BC等于D，则已完成，因BC已等于D。
But, if BC is greater than D, let CE be made equal to D, and with centre C and distance CE let the circle EAF be described; let CA be joined. Then, since the point C is the centre of the circle EAF, CA is equal to CE.
若BC大于D，在BC上截取CE等于D，以C为圆心、CE为半径作圆EAF，连接CA。
But CE is equal to D; therefore D is also equal to CA.
因C是圆EAF的圆心，CA等于CE；又CE等于D，故D等于CA，即CA为所求线段。