在新北师大版九上第四章总复习中,有两道较难的习题,一道需要做辅助线,一道要求三条线段的比例.习题1如图1,在ABCD中,对角线AC与BD相交于点O,在DC的延长线上取一点E,连接OE交BC于点F.已知AB=a,BC=b,CE=c,求CF的长.习题2如图2,四边形ABCD和四边形ACED都是平行四边形,点R为DE的中点,BR In the New Normal University Edition 9 on the fourth chapter of the general review, there are two more difficult exercises, one need to do auxiliary lines, a required ratio of three lines.Exercise 1 as shown in Figure 1, ABCD diagonal AC And BD intersect at point O, take a point E on the extension of DC, connect OE intersection BC at point F. Known AB = a, BC = b, CE = c, Quadrilateral ABCD and quadrilateral ACED are parallelogram, point R is the midpoint of DE, BR