(1-1/2^2)(1-1/3^2).(1-1/n^2)=（1-1/2)(1+1/2)(1-1/3)(1+1/3)……(1-1/n)(1+1/n)=(1-1/2)(1-1/3)……(1-1/n)(1+1/2)(1+1/3)……(1+1/n)=(1/2*2/3*3/4……n-1/n)(3/2*4/3*5/4……(n+1)/n)=1/n*(n+1)/2=(n+1)/2n=1/2+...