由于 dx dt =f″(t), dy dt =tf″(t)∴ dy dx = dy dt dx dt =t∴ d2y dx2 = d dx ( dy dx )= d dt ( dy dx ) dt dx = dt dx = 1 f″(t) ∴ d3y dx3 = d dt ( 1 f″(t) ) dt dx =? f″′(t) [f″(t)]3