(1-1/2^2)(1-1/3^2)(1-1/4^2)……(1-1/9^2)(1-1/10^2)=(1-1/2)(1+1/2)(1-1/3)(1+1/3)......(1-1/9)(1+1/9)(1-1/10)(1+1/10)=1/2×3/2×2/3×4/3×...×8/9×10/9×9/10×11/10=1/2×11/10=11/20
