曳物线,从曲线C上某一动点P的切线与某一定直线l的交点Q到点P的线段长恒为定值,则称曲线C为曳物线(tractrix)。直线l为其渐近线。
写法1:
vertices = 1000 t = from (-2*PI) to (2*PI) x = 2/[E^t+E^(-t)] y = t-tanh(t) x = limit(x, -20, 20) y = limit(y, -20, 20)
写法2:
vertices = 1000 t = from (-2*PI) to (2*PI) x = sech(t) y = t-tanh(t) x = limit(x, -20, 20) y = limit(y, -20, 20)
原文：http://www.cnblogs.com/WhyEngine/p/3839875.html