24.如圖1,等邊△ABC中,DE∥BA分別交BC、AC于點(diǎn)D、E.
(1)求證:△CDE是等邊三角形;
(2)將△CDE繞點(diǎn)C順時(shí)針旋轉(zhuǎn)θ(0°<θ<360°),設(shè)直線AE與直線BD相交于點(diǎn)F.
①如圖2,當(dāng)0°<θ<180°時(shí),判斷∠AFB的度數(shù)是否為定值,若是,求出該定值;若不是,說明理由;
②若AB=7,CD=3,當(dāng)B,D,E三點(diǎn)共線時(shí),求BD的長(zhǎng).