原式=(1-1/2)×(1+1/2)×(1-1/3)×(1+1/3)×(1-1/4)×(1+1/4)...(1-1/10)×(1+1/10)
=1/2×3/2×2/3×4/3×3/4×5/4.×9/10×11/10
=1/2×11/10
=11/20.