《几何》第二册P144有如下一段文字:“例水平放着的圆柱形排水管的截面半径是12cm,其中水面高为6cm,求截面上有水的弓形的面积(精确到1cm~2).解如图7—79,连结OA、OB,作弦AB的垂直平分线OD,垂足为D,交AB弧于点C,…”其中,“作弦AB的垂直平分线OD”不妥。为什么?“O”是已知圆圆心,即已知点;“AB”是已知弦,即已知线段,“作弦AB的垂直平分线OD”岂不是过已知点作已知线 “Geometry” Volume II P144 has the following paragraph: “The radius of the section of a horizontally-placed cylindrical drain pipe is 12 cm, of which the water surface height is 6 cm. Find the area of ​​the arch-shaped water on the cross section (accurate to 1 cm~2). Solution as shown in Figure 7-79, link OA, OB, for the vertical bisector OD of the string AB, the foot is D, the intersection AB arc at the point C, ... “wherein,” the vertical bisector OD of the string AB“ is not appropriate . Why? ”O“ is the known center of the circle, known as the point; ”AB“ is the known chord, known as the line segment, ”the vertical bisector OD of the chord AB" is not known as the known line