Build another bridge GF on the river bank to connect af, ga and BG.
∫DE is parallel and equals AA'
A quadrilateral is a parallelogram.
∴DA=EA'
Similarly: AF=A'G
The distance from B to A via DE is: BE+DE+DA=BE+AA'+EA'= segment BA'+AA'
The distance from B through FG to A is BG+GF+FA=BG+AA'+GA'= broken BGA'+AA'
The shortest line segment between two points
∴ paragraph ba'