原式=[1/（2-1）!-1/（2!）]+[1/（3-1）!-1/3!]+……+[1/(n-1)!-1/n!]
=1/1!-1/2!+1/2!-1/3!+.+1/(n-1)!-1/n!
=1-1/n!
=(n!-1)/n!