dx/dt=cost
dy/dt=sint+tcost-sint=tcost
y'=dy/dx=(dy/dt)/(dx/dt)=t
y"=d^2y/dx^2=d(y')/dx=d(y')/dt/(dx/dt)=1/cost
当t=π/4时,y"=1/cos(π/4)=√2
dx/dt=cost
dy/dt=sint+tcost-sint=tcost
y'=dy/dx=(dy/dt)/(dx/dt)=t
y"=d^2y/dx^2=d(y')/dx=d(y')/dt/(dx/dt)=1/cost
当t=π/4时,y"=1/cos(π/4)=√2