Let M be the midpoint of side AC, the line BM intersects the circle Ω again at point F′, and the line FM intersects the circle Ω again at point B′ (see Fig. 20).
Since arcs AE and CE are equal, points M and E lie on the perpendicular bisector ℓ of segment AC. Therefore, ∠EMD=90∘ and, consequently, M lies on the circle ω.
We have:
21B′C′E=∠B′FE=∠MFE=∠MDE=∠CDE=21(AB+CE)=21(AB+AE)=21BAE.
From the equality of arcs B′C′E and BAE, it follows that points B and B′ are symmetric with respect to ℓ, so lines BM and B′M (and therefore points F′ and F) are symmetric with respect to ℓ. The last statement means that EF=EF′, whence ∠FBE=∠F′BE. We obtain that lines BF and BM are symmetric with respect to line BE, which is what was required to be proved.
