1减二的平方分之一乘1减三的平方之一一直到n
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)...(1-1/n)(1+1/n)
=1/2x3/2x2/3x4/3x...x(n-1)/nx(n+1)/n
=1/2x(n+1)/n
=(n+1)/2n