dx/dt=-sin t + sin t + t cos t=t*cos t ①
dy/dt=cos t + cos t - t sin t=-t*sin t ②
②/①有:dy/dx=-tan t ③ 所以dy/dx|(t=pi)=-tan t |(t=pi)=0
③对t求导:d(dy/dx)/dt=-1/(cost)^2 ④
④/①有:d(dy/dx)/dt/(dx/dt)=d^2y/dx^2=-1/t*(cos t)^3
当t=pi 时d^2y/dx^2=-1/t*(cos t)^3=1/pi