A:for direction
B:for directions
C:with direction
D:with directions
下列投影一定不会改变△ABC的形状和大小的是( )
如图,已知四边形ABCD内接于圆,对角线AC与BD相交于点E,F在AC上,AB=AD,∠BFC=∠BAD=2∠DFC.
(1)若∠DFC=40°,求∠CBF的度数;
(2)求证:CD⊥DF.