[tex]\it a)\\ \\ \left.\begin{aligned} ABCD\ -\ paralelogram\ \Rightarrow\ BC=AD\\ \\ Dar,\ din\ enun\c{\it t},\ avem\ rela\c{\it t}ia\ BC=CE \end{aligned} \right \}\ \Rightarrow AD=CE\ \ \ \ (1)\\ \\ \\ Din\ enun\c{\it t}\ \Rightarrow\ DE||AC\ \ \ \ (2)\\ \\ (1),\ (2) \Rightarrow ACED\ -\ trapez\ isoscel\\ \\ AE=CD\ (diagonalele\ trapezului\ isoscel)\ \ \ \ (3)\\ \\ Dar,\ CD=AB\ (laturi\ opuse\ ale\ paralelogramului)\ \ \ \ (4)\\ \\ (3),\ (4)\ \Rightarrow AB=AE \Rightarrow [AB]\equiv[AE][/tex]