由　　　lim(h→0)[(x+h)^(1/6) - x^(1/6)]/h　　= lim(h→0)1/{[(x+h)^(5/6) + [(x+h)^(4/6)[x^(1/6)] + [(x+h)^(3/6)][x^(2/6)] + … + [x^(5/6)]}　　= 1/[6x^(5/6)]即得.