(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)……(1-1/10)(1+1/10)=(1/2)(3/2)(2/3)(4/3)……(9/10)(11/10)中间约分约掉了=(1/2)*(11/10)=11/20
