people_and_dog

代码说明：

  一个慢跑者在平面上沿着他喜欢的路径跑步，突然一只狗攻击他，这只狗以不变的速度跑向跑步者，狗的方向始终指向跑步者，计算并画出狗的运动轨迹（如需其他条件，请自己假定）.(A jogger in the plane along the path he likes running, a dog suddenly attacked him, the dog ran at constant speed running, the dog always point to the direction of running, and calculate and draw the dog)