证明共线的四条线段的等积式，一般都要进行代换．本文列举用不同形式代换的五种方法．一、利用相等的线段代换例１如图１，过圆心O的直线l垂直于弦AB，交⊙O于D、M两点，作⊙O的另一条弦AE，并延长交l于点C，连结BE交DM于点F．求证：OD２＝OC·OF．分析：OD是⊙O的半径，可用 It is proved that the isomorphism of the four line segments of the collinear line is generally replaced. This article lists five methods for substitution in different forms. First, the use of an equivalent line segment substitution example 1, as shown in Figure 1, over the center of the line l is perpendicular to the string AB, the intersection of O and D, M two points, for the other string of AE AE, and extend the intersection of l and the point C, link BE to DM at point F. Proof: OD2=OC·OF. Analysis: OD is the radius of ⊙O, available