请用视频讲解第13题---Question Stem: 13、如图，在等边△ABC中，点D、E都在边BC上，∠DAE=30°，DE=7．若BD：EC =3：5，则AD的长为_______． Geometric Figure Description: Type: Geometric figure (Triangle) Main Elements: - Vertices: A, B, C. Arranged as an equilateral triangle with base BC horizontal. Vertex A is above BC. - Sides: AB, BC, AC. Sides AB and AC are marked with double dashes, implying they are equal. - Angles: ∠B is labeled as 60°. ∠DAE is marked with an arc and labeled as 30°. - Points: D and E are on the segment BC. D is positioned to the left of E. - Segments: AD, AE, DE. Segment DE is labeled with length 7. Segment BD is labeled as 3x. Segment EC is labeled as 5x. Segments AD and AE are marked with single dashes, implying they are equal. - Labels and Annotations: "30" near ∠DAE, "7" near DE, "60°" near ∠B, "3x" near BD, "5x" near EC. The caption below the diagram is "第13题图". - Additional handwritten notes: There are handwritten annotations and sketches near the diagram, including a point labeled B', segments labeled "√3X", "1.5X", "1.5X√3", and an angle labeled "60°". Extracted Text: 的 长 为 13、如图，在等边△ABC中，点D、E都在边BC上，∠DAE=30°，DE=7．若BD：EC =3：5，则AD的长为_______． B 3X D 7 E 5X 第13题图