：(1-1/2²)(1-1/3²)···(1-1/9²)(1-1/10²）
=(1-1/2)(1+1/2)(1-1/3)(1+1/3)·····(1-1/10)(1+1/10)
=1/2×3/2×2/3×4/3×···×9/10×11/10
=1/2×11/10
=11/20