=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)……(1-1/12)(1+1/12)
=1/2×3/2×2/3× 4/3×3/4 ×……× 11/12 ×13/12
=1/2×13/12
=13/24