Just Right:If either {$\ a,c<b<a+c\ $} or {$\ b,c<a<b+c\ $}, then the path on the {$a\times b\times c$} box returns to the start, to the black corner. Proof: The case for {$\ a,c<b<a+c\ $} appeared when we described rolling over the plane.  The circled intersection happens where it happens because {$a<b$}. The next intersection is where it is because {$b<a+c$}.  And the next because {$a+c<a+b$}, or {$c<b$}.  The final corner is reached because {$b+a+c=a+c+b$}  The situation for {$\ b,c<a<b+c\ $} is similar. |