=(1-1/2)(1+1/2)(1-1/3)(1+1/3)(1-1/4)(1+1/4)(1-1/5)(1+1/5)(1-1/6)(1+1/6)=1/2 × 3/2×2/3 × 4/3×3/4 ×5/4×4/5 ×6/5×5/6 ×7/6=1/2×7/6=7/12
