tanθ=0.5t/10求导sec^2θdθ/dt=0.05转动速度=dθ/dt=0.05cos^2θ0.5t=15,t=30,tanθ=15/10=1.5=3/2cosθ=2/√(2^2+3^2)=2/√13dθ/dt=0.05cos^2θ=0.05（2/√13）^2=0.05x4/13=0.2/13=1/65(rad/s)
