题目 已知如下左图,矩形ABCD中,AB=5,AD=20,点M分BC为BM:MC=1:2,DE垂直于AM,E为垂足。求DE的长。 (1993,天津市中考试题) 此题是由《平面几何》第二册P52第8题“已知矩形ABCD中,AB=a,BC=b,M是BC的中点,DE⊥AM,E是垂足(如上右图)。求证:DE=2ab/(4a~2+b~2)~(1/2)”经特殊化、改变条件、推 Title It is known that the left picture is as follows. In the ABCD of the rectangle, AB=5, AD=20, point M and BC are BM:MC=1:2, DE is perpendicular to AM, and E is a sag. Find the length of DE. (1993, Tianjin Middle School Exam Questions) This question is from the “Plane Geometry” Volume II P52 Question 8 "known rectangle ABCD, AB = a, BC = b, M is the midpoint of BC, DE ⊥ AM, E It is pedantic (as shown in the right figure above.) Proof: DE=2ab/(4a~2+b~2)~(1/2)” Specialized, changed condition, pushed