n!*(n+2) = n*n!+ n!* 2 = (n+1)!+ n!所以1!*3 = 2!+ 1!-2!*4 = -3!- 2!3!*5 = 4!+ 3!-4!*6 = -5!-4!.2011!*2013 = 2012!+2011!-2012!*2014 = -2013!-2012!2013!*2015 = 2014!+2013!相加得1!*3-2!*4+3!*5-4!*6+...-2...