帮我讲解一下这道题---```text 20. 如图, 在△ABC 中, D 为 BC 中点, 延长 BA 至点 E, 使 AE = AB, 延长 DA 至点 F, 使 AF = AD, 连接 CE, EF. (1)求证: 四边形 CDEF 是平行四边形; (2)若 EB 平分 ∠CEF, tanB = 4/3, AB = 1, 求 AC 的长. Chart/Diagram Description: Type: Geometric figure. Main Elements: - Triangle ABC. - Point D is on the line segment BC, specifically the midpoint of BC. - Point E is on the extension of line segment BA through A, such that A is the midpoint of BE (collinear E, A, B with AE = AB). - Point F is on the extension of line segment DA through A, such that A is the midpoint of DF (collinear F, A, D with AF = AD). - Line segments CE and EF are drawn. - Labels: Points A, B, C, D, E, F are labeled. - Relative Position: A is inside or on the boundary of the quadrilateral formed by B, C, E, F. B, D, C are collinear in that order. E, A, B are collinear in that order with A between E and B. F, A, D are collinear in that order with A between F and D. ```