1的阶乘1!=1x1=1; 2的阶乘2!=2x1=2; 3的阶乘3!=3x2x1=6; 4的阶乘4!=4x3x2x1=24; 5的阶乘5!=5x4x3x2x1=120; 6的阶乘6!=6x5x4......