Two poles AB and CD are 20m high and 100m apart.
A rope is suspended between B and C,
and it takes the shape of a parabola under its own weight.
the maxium sag at the middle of the rope is 10m.
How high is the rope above point X, given that XD= 25m?
左邊D文字打哂..右下角就係張圖...虛線我自己點上去-.-