:(1-1/2^2)(1-1/3^2)(1-1/4^2)L(1-1/9^2)(1-1/10^2)L(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 ×…… ×(1+1/n)
=1/2×(n+1)/n
=(n+1)/2n