Answer:
On the diagram, construct lines BY, XY and DY.
Now, consider triangles ΔXBY and ΔXDY. We can say that these triangles are congruent by the SAS postulate (they share side XY, XY bisects ∠BYD so ∠BYX=∠DYX, and BY=DY=radius). If they're congruent, their sides are the same so BX=DX.
BX=DX
AB+radius = CD +radius
AB=CD
