An old chestnut
goes like this:
There is a right-angled corner in a long hallway in a building, A meters wide in one wing of the hallway and B meters wide in the other wing. How long a straight ladder can be carried lengthwise around the corner without bending or breaking it? Ignore the thickness of the ladder, but treat the ladder as being so wide and the ceiling so low that you'll do best carrying the ladder parallel to the ground.