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