Side CD of parallelogram ABCD is twice as large as side BC. Point F is the middle of the side of the CD. Prove that BF is the bisector of the angle ABC
BC = CD / 2 = CF (by condition of the problem)
Therefore the triangle BCF is isosceles.
By the property of an isosceles triangle:
∠CFB = ∠CBF
∠CFB = ∠ABF (since these are cross-angles)
It turns out that ∠CBF = ∠ABF
Therefore, BF is a bisector.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.