影子y=400cotθ
影子移动速度v=dy/dt=(400cosθ/sinθ)′dθ/dt=400(-sin^2θ-cos^2θ)/sin^2θ*dθ/dt=-400/sin^2θ*(-0.25)
=100/sin^2θ(rad/h)
∴θ=π/6时,v=100/(1/2)^2=400rad/h