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!