What similar (proportional) proof questions should be used as auxiliary lines, and if so, how to do it? What is the practice of auxiliary line for junior high school mathematical geometry problems?
Proof: pass d as DF//AC. In triangle BCE, D is the midpoint of BC, so DF= 1/2CE, AE: EC = 1: n, so AE: DF = 2: n ... Because triangle AOE is similar to triangle DOF, AO:OD = AE；; DF=2:n