Point D [located at (0,0)] is the bottom left corner (vertex) of the lake..... point "E" [located at 20, 20] is the lower left vertex of the island.....between D and E is the water......and AB is the plank laid across the water anchored at both ends on dry land.....
The second plank - 19 ft long - spans CE = the distance [ about 18.78 ft ] from the middle of the first plank to the lower left vertex of the island
