dx/dt=f''(t)
dy/dt=f'(t)+tf''(t)-f'(t)=tf''(t)
dy/dx=(dy/dt)/(dx/dt)=1/t
d^2y/dt^2=f''(t)+tf'''(t)
d^2y/dx^2=(d^2y/dt^2)/[(dx/dt)*(dx/dt)]=1/f''(t)+tf'''(t)/[f''(t)^2]
dx/dt=f''(t)
dy/dt=f'(t)+tf''(t)-f'(t)=tf''(t)
dy/dx=(dy/dt)/(dx/dt)=1/t
d^2y/dt^2=f''(t)+tf'''(t)
d^2y/dx^2=(d^2y/dt^2)/[(dx/dt)*(dx/dt)]=1/f''(t)+tf'''(t)/[f''(t)^2]