50*(1/50-1）*(1/49-1)*(1/48-1)…（1/3-1）*（1/2-1）/50
=(-49)*(1/49-1)*(1/48-1)…（1/3-1）*（1/2-1）/50
=2*(1/2-1)/50
=-1/50
括号内为偶数前边为正,奇数前为负,所以1/2前卫正2结果应为负.