Okay so the Percution problem is this... sorry for my english
A destroyer is in the middle of a heavy fog that vanishes for instance and lets you see an enemy submarine in the surface at 4km of distance.
a) The submarine immersed instantly and moves on to an unknown direction.
b) The destroyer travels 3km in straight line to the submarine.
What trajectory should the destroyer fallow to be sure that it will pass directly over the submarine, if its speed v is three times the submarines?
Answer is: r = e^(θ/√8)