用平方差 =(1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/n)(1+1/n) =(1/2)(3/2)(2/3)(4/3)……(n-1)/n*(n+1)/n 中间约分 =(1/2)*(n+1)/n=(n+1)/(2n)
