dy/dx
= d[tf'(t)-f(t)] / d(f'(t))
= [f'(t)+tf''(t)-f'(t)]dt / f''(t) dt
= tf''(t) / f''(t)
= t
d(dy/dx)/dx
= d(t) / d(f'(t))
= dt / f''(t)dt
= 1/f''(t)
d^3 y /dx^3
= d(d^2y)/dx
= d(1/f''(t))/d(f'(t))
= [-f'''(t)/f''^2(t)]dt/f''(t)dt
= - f'''(t)/[f''(t)]^3